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The apparent change in direction of a celestial source of light caused by an observer's component of motion perpendicular to the impinging rays. In astronomy, aberration manifests, for example, as a star appearing aberrantly from what its true…

A common medicine used to relieve mild to moderate pain and reduce fever. Acetaminophen (N-acetyl-p-aminophenol), or paracetamol, is a white crystalline compound with the molecular formula C8H9NO2 (Fig. 1). It is a member of the aniline family of…

Technologies to optimize the performance of optical systems wherein controllable optical elements maintain a sharply focused image in the presence of wavefront aberrations. The practical development of adaptive optics started in the late 1960s. Main…

A disease of the nervous system characterized by a progressive dementia that leads to profound impairment in cognition and behavior. Dementia occurs in a number of brain diseases where the impairment in cognitive abilities represents a decline from…

A member of a group of organic compounds that can be considered as derived from ammonia by replacement of one or more hydrogens by organic functional groups. Generally amines are bases of widely varying strengths, but a few that are acidic are…

One of the many hundreds of thousands of small planets (technically called minor planets) revolving around the Sun, mainly between the orbits of Mars and Jupiter. Asteroids are rocky bodies varying greatly in size from less than 10 meters across (33…

An analytical technique for determining the composition of a sample in terms of the chemical elements present and their quantities or concentrations. Unlike other methods of elemental analysis, however, the sample is decomposed into its constituent…

A neurodevelopmental condition that impairs the way that a person relates to and communicates with other people. Persons with autism have difficulties with social communication and interaction. They also can have unusual behaviors, such as…

A kind of mathematics whose variables can only be True or False. A variable is called Boolean if it can only hold two possible values: True or False. The value True can be represented by the number one and False by zero. For example, if A is a…

A poisonous colorless, odorless, tasteless gas (CO), resulting from the incomplete combustion of carbon-containing materials. Carbon monoxide (Fig. 1) is an indoor hazard causing hundreds of deaths annually, and it is one of the six major air…

Light emitted by a high-speed charged particle when the particle passes through a transparent, nonconducting, solid material at a speed greater than the speed of light in the material. The emission of Cerenkov radiation—also spelled Cherenkov—is…

The attractive force that holds atoms or ions together. Chemical bonds are very strong. To break one bond in each molecule in a mole of material typically requires an energy of many tens of kilocalories.It is convenient to classify chemical bonding…

A chemical element, Cl, atomic number 17 and atomic weight 35.453. Chlorine exists as a greenish-yellow gas at ordinary temperatures and pressures (see illustration). It is second in reactivity only to fluorine among the halogen elements, and hence…

A psychotherapy with roots in both cognitive science and behavioral science. Cognitive-behavioral therapy (CBT) is the most widely used evidence-based psychotherapeutic practice for a variety of mental health disorders, including anxiety, depression…

The spinning of the Earth upon its planetary axis and movement of the Earth around the Sun. While appearing constant, the Earth's rate of rotation, where one complete rotation is measured as a day, and the rate of the Earth orbital motion around the…

One of the two places in the sky where the ecliptic crosses the celestial equator, or one of the two times of the year when the Sun crosses these points. The ecliptic is the great circle across the sky that marks the mean path of the Sun; the…

A mathematical system that details the properties and relationships of spatial objects such as points, lines, shapes, and planes. Euclidean geometry is also known as flat geometry or planar geometry, because planes in Euclidean geometry have the…

Planets beyond our solar system. For millennia, humankind has known about most of the major planets in our solar system because these worlds are visible to the naked eye. Exoplanets, however, were not indisputably known until 1995, when astronomers…

The use of pollen and spore evidence in legal cases. In its broader application, the field of forensic palynology also includes legal information derived from the analysis of a broad range of microscopic organisms that can be found in both fresh and…

In thermodynamics, the amount of useful work that can be extracted from a closed thermodynamic system. The Helmholtz free energy applies to a process that takes place at a constant temperature T (isothermal process). The Gibbs free energy applies to…

A space telescope mission dedicated to detecting exoplanets. The tenth mission launched under the National Aeronautics and Space Administration's (NASA's) Discovery Program, Kepler was the agency’s first mission dedicated to the search for planets…

A salt or ester of lactic acid (CH3CHOHCOOH). In lactates, the acidic hydrogen of the carboxyl group has been replaced by a metal or an organic radical. Lactates are optically active, with a chiral center at carbon 2. The two optical isomers of a…

The inverse function of an exponential function in mathematics. Stated less technically, a logarithm is the opposite of an exponent. Logarithms are used to solve equations in which the variable is an exponent. Historically, tables of logarithms…

The planet fourth outward from the Sun and the planet most like Earth in terms of geology, climate, and suitability for life. Mars is nicknamed the "Red Planet" for its reddish hue (Fig. 1). Besides Earth, Mars is the most-studied world in the solar…

An analytical technique for identification of chemical structures, determination of mixtures, and quantitative elemental analysis by measuring the mass-to-charge (m/z) ratio of ions produced when a sample is ionized. Determination of organic and…

A device that is capable of converting chemical energy into electrical energy with the use of whole bacterial cells as the catalyst. Fuels that are produced biologically are known as biofuels. Biofuel cells convert chemical energy from organic…

A chemical element, Os, atomic number 76, atomic weight 190.2. Osmium is a hard white metal of rare natural occurrence, and is usually found in nature alloyed with other platinum metals. Metal Periodic table Platinum The periodic table of the…

A two-component reaction involving the loss of electrons (oxidation) by one chemical species and the gain of electrons (reduction) by another; also known as a redox reaction. Both oxidation and reduction occur simultaneously and in equivalent…

A slow change in the direction of the axis of rotation of the Earth as well as a very small change in the orientation of the Earth's orbit around the Sun. As a consequence of the precession of equinoxes, both the celestial equator (the projection of…

A branch of mathematics involving numerical measures of how likely it is that a particular event will occur. A probability is a number between 0 and 1. An event that will never occur has a probability of zero, and an event that is certain to occur…

The study of chemical changes resulting from the absorption of high-energy ionizing radiation. Such radiation includes alpha particles, electrons, gamma rays, fission fragments, protons, deuterons, helium nuclei, and heavier charged projectiles.…

In medicine and health sciences, a value expected for an individual whose state of health is known and well described, and the interval of values broadly expected for a population, respectively. Results of clinical laboratory tests and other…

The second largest planet in the solar system and the sixth in order of distance from the Sun. The outermost planet known prior to 1781 CE, Saturn is surrounded by a system of rings far more extensive than any other in the solar system (Fig. 1), as…

The field of knowledge concerned with collecting, analyzing, and presenting data. Statistical methods are regularly used by researchers in the physical, biological, and social sciences, as well as engineers, business managers, government officials…

An instrument used to collect, measure, or analyze electromagnetic or other radiation from distant objects. A telescope overcomes the limitations of the human vision by increasing the ability to see faint objects and discern fine details. In…

One of the elements of atomic numbers 21–31, 39–49, and 71–81. The symbols of these elements, along with their atomic numbers and valence-shell electronic configurations, are given in Fig. 1. The elements are arranged in the order in which they…

A chemical obtained from wood. Woody plants comprise the greatest part of the organic materials produced by photosynthesis on a renewable basis. Replacement of fossil hydrocarbons, from which most organic chemicals are derived, is possible in many…

The scattering of x-rays by matter with accompanying variation in intensity for specific directions (or wavelengths) due to interference effects. X-ray diffraction is a powerful and readily available method for determining atomic arrangements in…

A plant, Musa textilis, of the banana family that is valuable for its hard fiber (also known as Manila hemp). Abaca (Musa textilis) is a flowering plant native to the Philippines and Borneo. Taxonomically, it is assigned to the order Zingiberales.…

A major plant hormone that plays an important role in plant growth and development. Abscisic acid (ABA), with a chemical formula of C15H20O4 (Fig. 1), is one of the most significant hormones found in plants. It is responsible for regulating various…

The study of systems consisting of arbitrary sets of elements of unspecified type, together with certain operations satisfying prescribed lists of axioms. Abstract algebra has been developed since the mid-1920s and has become a basic idiom of…

An increase in the expansion rate of the universe, attributed to dark energy. In 1998, astronomers presented evidence that the universe's expansion is not slowing down, as would be expected due to the gravity from the abundances of matter strewn…

Habituation to a specific practice, such as drinking alcoholic beverages or using drugs (for example, heroin, cocaine, or amphetamines), and disorders characterized by the chronic use of a drug, alcohol, or similar substance. Addiction and addictive…

Any disorder of recognition. Agnosia is a rare neurological condition in which an individual fails to recognize and identify persons or objects (see illustration). Typically, agnosia is the result of damage to specific areas of the brain.…

The repeated and/or excessive use of alcohol (ethanol) that continues despite negative physical, psychiatric, or social consequences. Alcoholism is characterized by consuming alcohol more often, in larger amounts, or over longer periods than…

A useful carbon-carbon bond-forming reaction in which aldehyde and alcohol functional groups are observed during the reaction. The aldol reaction was first reported by Aleksandr Porfiryevich Borodin, who observed the reaction using pentanal and…

An informal assemblage of predominantly aquatic eukaryotes that carry out oxygen-evolving photosynthesis, but lack specialized water-conducting and food-conducting tissues. Algae (Fig. 1) are eukaryotic organisms (having an organized nucleus).…

A significant, yet relatively selective loss of memory. Amnesia is a type of memory loss, often resulting from brain disease or injury (see illustration). The inability to remember can be temporary or permanent. In general, amnesia can be…

A statistical technique that partitions the total variation in experimental data into components assignable to specific sources. Analysis of variance is applicable to data for which (1) effects of sources are additive, (2) uncontrolled or…

A disorder of language acquired as a result of brain disease. Aphasia is impairment in the use of spoken or written language caused by brain disorders (see illustration) that cannot be explained by paralysis or incoordination of the articulatory…

An impairment in the performance of voluntary actions despite intact motor power and coordination, sensation and perception, and comprehension. Apraxia is a neurological condition in which there is difficulty with skilled tasks or learned movements…

The study of the universe and the objects in it through scientific investigation. The goal of astronomical research is to understand celestial objects and the nature and evolution of the cosmos (Fig. 1). Because much of contemporary astronomy uses…

The study of the stars, galaxies, and other objects outside of the Solar System, with attention to physical causes. Astrophysics developed in the late 19th century, taking over from the mere mapping of positions of objects that had been the…

A chemical reaction or series of reactions leading to the predominant or exclusive formation of a single enantiomer, that is, one of an isomeric pair of chemical compounds whose molecules are nonsuperimposable mirror images. Enantiomers are said to…

A device that uses an internal resonance frequency of atoms (or molecules) to measure the passage of time. Atomic clocks (Fig. 1) are relied on internationally as the most accurate time and frequency standards with myriad uses in commerce, industry,…

A common psychiatric disorder (especially in children) characterized by attentional difficulties, impulsivity, and hyperactivity. Attention deficit hyperactivity disorder (ADHD) is a mental disorder marked by an individual's inability to pay…

Any of a class of plant growth substances (phytohormones or plant hormones) that have an essential role in coordination of many growth and developmental processes during the life cycle of plants, especially the ability to increase the rate of shoot…

A class of nonflagellate unicellular algae, commonly called diatoms, with boxlike silicified walls. Members of the algal class Bacillariophyceae are commonly termed diatoms (Fig. 1). Diatoms range in maximum dimension from 4 μm (0.00004 in.) to more…

Any of several leguminous plants, or their seeds, long utilized as food by humans or livestock. There are more than 40,000 varieties of beans, with only a small number of these being mass-produced for human or animal consumption. In particular,…

The drug as well as the plant known as deadly nightshade (Atropa belladonna). The belladonna plant (Atropa belladonna; see illustration) belongs to the nightshade family, Solanaceae, in the order Solanales. The plant is a coarse and poisonous…

An aromatic hydro­carbon, molecular formula C6H6, also know as benzol. Benzene is a clear, colorless, and flammable liquid which boils at 80.1°C and freezes at 5.5°C.Benzene (1) is an important chemical intermediate used to produce ethylbenzene…

The theory that the universe began in a hot, dense state and has since expanded and cooled. The big bang is the generally accepted cosmological theory for the origin, properties, and evolution of the universe (Fig. 1). Cosmology Universe 1 Timeline…

A discipline that combines biology, mathematics, and computer science to acquire, store, analyze, and disseminate complex biological data. The term bioinformatics was coined by Paulien Hogeweg and Ben Hesper in the early 1970s to describe the "the…

Fungi that exhibit the natural phenomenon of bioluminescence (the ability of an organism to emit visible light). Bioluminescence is the emission of photons of visible light by living organisms via the chemical reaction of molecular oxygen (O2) with…

An integrated device consisting of a biological recognition element and a transducer capable of detecting the biological reaction and converting it into a signal that can be processed. Ideally, the sensor should be self-contained, so that it is not…

A major mental disorder in which there are lifelong episodes of both mania and depression; also known as manic-depressive illness. Bipolar disorder, also termed manic-depressive illness or manic depression, is characterized by sudden and often…

A region of spacetime exerting a gravitational field so strong that neither matter nor radiation can escape. Black holes are extreme cosmic objects predicted by German-born U.S. physicist Albert Einstein's theory of general relativity. Within a…

A mental health illness characterized by a compulsive or obsessive focus on a perceived flaw in appearance. Body dysmorphic disorder (often abbreviated BDD) is a psychiatric illness in which a person has a severe, excessive preoccupation with a…

A garden for the culture of plants collected chiefly for scientific, educational, and esthetic purposes. A botanical garden (see illustration) is an establishment in which various plants, including trees, are collected, cultivated, and grown for…

The branch of biological science that embraces the study of plants and plant life. Botany is the scientific study of plants (see illustration), encompassing investigations of plant anatomy, physiology, genetics, phylogeny, ecology, pathology, and…

A division that consists of small and relatively simple plants commonly known as mosses, peat mosses, granite mosses, liverworts, and hornworts. Members of the division Bryophyta are referred to as bryophytes (see illustration). These nonvascular…

In its simplest form, a vector is a directed line segment. Physical quantities, such as velocity, acceleration, force, and displacement, are vector quantities, or simply vectors, because they can be represented by directed line segments. The algebra…

The measurement of the quantity of heat transferred during chemical reactions, changes of state, mixing of substances, as well as other chemical and physical processes. The unit of energy in the International System of Units is the joule, symbolized…

A genus comprising yeasts that cause a variety of human diseases referred to as candidiasis. All Candida species are single-celled yeasts (fungal organisms) that are ovoid in shape and 3–5 μm in size, and they multiply by a process called budding.…

An order of flowering plants (angiosperms) including the mustard and caper families. The plant order Capparales contains various families of eurosid dicotyledons. Most notably, mustards, capers, and various ornamental plants (see illustration) are…

A member of a class of common pigments, typically of yellow, orange, or red hues, that serve in both light absorption and the protection against too much light in photosynthesis as well as in the vision process. Carotenoids comprise a class of…

A branch of physical chemistry that seeks to explain time-dependent phenomena, such as energy transfer and chemical reaction, in terms of the detailed motion of the nuclei and electrons that constitute the system. As such, chemical dynamics seeks…

A type of sweet or sour fruit of the Prunus genus, as well as the tree that bears the fruit. Cherry trees belong to the Prunus genus in the order Rosales, and the sweet and sour fruits of these trees are called cherries (Fig. 1). There are…

A white or colorless amorphous polysaccharide that forms a base for the hard outer integuments of crustaceans, insects, and other invertebrates. Chitin (see illustration) is an important nitrogenous polysaccharide that is found abundantly in nature.…

A large palm, Cocos nucifera, widely grown throughout the tropics and valuable for its fruit and fiber. The coconut (Cocos nucifera) belongs to the plant family Arecaceae. The name coconut can refer to the tree and its fruit. Usually found near the…

Any of various tropical trees belonging to the genus Cola. Members of the genus Cola (family Malvaceae, order Malvales), which are native to tropical Africa, are called cola trees (see illustration). In particular, two species, C. acuminata and C.…

A natural and extremely useful extension of the familiar real numbers. They can be introduced formally as follows. Consider the two-dimensional real vector space consisting of all ordered pairs (a1, a2) of real numbers. Geometrically this space can…

Any living or fossil member of the plant division Pinophyta, consisting chiefly of woody trees or shrubs that produce cones and evergreen needles. Conifers are predominantly cone-bearing, evergreen trees and shrubs belonging to the order Pinales, as…

A fiber obtained from the cotton plant Gossypium. The cotton plant (genus Gossypium, order Malvales; Fig. 1) has been used by humans for more than 5000 years. The fiber (cotton) obtained from this plant is the most widely used natural fiber because…

A curve traced in the plane by a point on a circle that rolls, without slipping, on a line. If the line is the x axis of a rectangular coordinate system, at whose origin O the moving point P touches the axis, parametric equations of the cycloid are…

A woody higher plant that regularly loses its leaves at the end of each growing season. Deciduous plants drop (shed) their leaves seasonally, most commonly during the fall (autumn). In general, the dropping of leaves occurs at the inception of an…

A syndrome characterized by a generalized decline in cognition that is severe enough to cause functional impairment in daily activities. Dementia is a term used to describe a collection of symptoms of severe cognitive decline. Individuals suffering…

A heterogeneous affective disorder involving a depressed mood and/or a loss of interest in daily activities. The most common form of affective disorder is depression (also termed clinical depression) [Fig. 1]. A major depressive episode is defined…

A cycloaddition reaction between a diene and a dienophile. The simplest Diels-Alder reaction (O. Diels and K. Alder, 1928) occurs between butadiene and ethylene and is often referred to as a [4+2] cycloaddition. The numbers, four and two in…

A branch of mathematics that deals with intrinsic properties of curves and surfaces in three-dimensional Euclidean space. The intrinsic properties are those which are independent of the geometrical object's orientation or location in space. The…

The transport of matter from one point to another by random molecular motions. Diffusion occurs in gases, liquids, and solids.Diffusion plays a key role in processes as diverse as permeation through membranes, evaporation of liquids, dyeing textile…

In arithmetic and algebra, the process of finding one of two factors of a number (or polynomial) when their product and one of the factors are given. The symbol ÷ now used mostly in elementary English and American arithmetics to denote division…

A system that delivers controlled amounts of a therapeutic agent over a prolonged period or in a desirable temporal fashion, thereby maintaining required plasma or tissue drug levels. Drug delivery systems are bioengineered mechanisms or devices…

Impairment of the ability to read. Developmental dyslexia, which is a learning disorder with a neurobiological basis, is characterized by unexpected difficulty in reading in children and adults who otherwise possess the intelligence and motivation…

The number e usually defined as the limit approached by the expression as n approaches infinity. If the given expression is expanded by the binomial theorem, and if one uses the theorem that the limit of the quotient of two polynomials of equal…

The controlled induction of a grand mal seizure under anesthesia to treat individuals suffering from certain psychiatric illnesses. Electroconvulsive therapy (ECT; Fig. 1) was introduced as a treatment for psychiatric disorders by the Italian…

A major clade or natural group of plants, comprising the land plants. The members of the Embryobionta or Embryophyta are commonly termed embryobionts or embryophytes, and they constitute the land plants. Embryophytes form a well-marked clade (a…

A volatile, fragrant oil obtained from plants. Essential oils are distinguished from those known as fixed oils, which are mainly triglycerides of fatty acids. Essential oils have been obtained from thousands of plants (Fig. 1) and are designated and…

The simplest cyclic ether or epoxide, with the formula C2H4O. Ethylene oxide is also called epoxyethane and oxirane, and has the below structure. Ethylene oxide was discovered in 1859 by C. A. Wurtz. His preparation from ethylene chlorohydrin and…

A former order of rosid flowering plants, division Magnoliophyta (Angiospermae), in the class Magnoliopsida (dicotyledons). The order Euphorbiales consisted of the large family Euphorbiaceae (with about 7500 species) and 3 small satellite families…