Search Results

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…

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

Earth's oldest desert and the most arid place outside of Antarctica's Dry Valleys. Located along the edge of the Pacific Ocean and to the west of the Andes in Chile's Antofagasta Province, the Atacama Desert (Fig. 1) has long been recognized as…

The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence.…

Any of the cells within a tumor that possess the capacity to self-renew and cause the heterogeneous lineages of cancerous cells that comprise the tumor. Cancer stem cells (CSCs) typically represent a small percentage of all cells in a tumor (rarely…

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 administration of chemical substances for the treatment of disease, especially cancer, or for the prevention of aberrant growth of cells or tissues (neoplasms). In general, chemotherapy is the use of chemicals to treat any disease, but the term…

The response of corals to environmental stress in which they eject the single-cell algae that live within the transparent coral tissue, making the white coral skeleton visible. Corals (Fig. 1) are animals that live in a partnership (symbiosis) with…

A genome-engineering technique that allows precise targeting of specific stretches of genetic code and editing of DNA at designated locations. The CRISPR/Cas9 system has been used as a general tool to engineer the genomes of many different…

The interaction of the behavior and ecology of hosts with the biology of pathogens as relating to the impact of diseases on populations. Although human diseases have been studied from an ecological perspective for centuries, diseases in plant and…

A radar system used to measure the relative velocity of targets and their position. The operation of these systems is based on the fact that the Doppler frequency shift in the target echo is proportional to the radial component of target velocity.…

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…

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…

Typically, an aquatic vertebrate with scales, gills, and paired fins. Fishes (Fig. 1) are found throughout the world's waterways, from saltwater oceans to freshwater streams and ponds. About 35,000 species of fishes have been described to date. Many…

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…

A plant that has altered genetic material (DNA) through recombinant DNA (rDNA) manipulation. Genetic engineering of plants allows for very specific genetic changes to be made to a plant's deoxyribonucleic acid (DNA), as opposed to the random…

A commercially grown plant that has altered genetic material through artificial recombinant manipulation. Genetic modification is the latest scientific tool for developing improved crop varieties. Such genetically modified (GM) crops can help to…

A biological organism whose genetic material has been manipulated artificially through genetic or genomic editing techniques. In contrast to the long history of artificial selection (domestication), in which humans selectively breed organisms to…

An ordered, internally consistent, internationally recognized sequence of time intervals, each distinct in its own history and record of life on Earth, including the assignment of absolute time in years to each geologic interval. The geologic time…

Significant and persistent changes in the state of Earth’s climate. Earth's past climates, or paleoclimates, have alternated between ice ages and periods warmer than today over relatively long time scales (millions to tens of millions of years)…

The rise in the surface temperature of the Earth. Global warming is one of the names given to the systematic increase of globally averaged surface temperature that has been measured since the mid-nineteenth century. There is abundant evidence that…

A phaneritic (visibly crystalline) plutonic rock composed chiefly of sodic plagioclase (oligoclase or andesine), alkali feldspar (microcline or orthoclase, usually perthitic), quartz, and subordinate dark-colored (mafic) minerals (biotite,…

A strongly cemented or hardened (indurated) dark gray sandstone that is characterized by abundant dark-colored detrital rock fragments and more than 15% clay minerals (matrix) between sand grains. The term graywacke evolved from initially informal…

The most common sulfate mineral, characterized by the chemical formula CaSO4 · 2H2O; it shows little variation from this composition.Gypsum is one of the several evaporite minerals. This mineral group includes chlorides, carbonates, borates,…

Pathologies or aberrations in structure or function of blood cells or the blood-clotting mechanism. Hematologic disorders affect the blood and blood-forming organs. Although many diseases and abnormal conditions may be reflected by the blood and its…

A metamorphic rock that has been subjected to heating during contact metamorphism around intrusive igneous rocks. Hornfels is typically fine-grained, although where it is subjected to high temperatures, large crystals called porphyroblasts can form.…

A species translocated from one region or continent to another outside its normal range, particularly when the translocation is the result of human activity (intentional or otherwise). Many thousands of species, including animals, plants, fungi,…

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…

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…

A procedure used to remove toxins, fluids, and other substances from the body when the kidneys are unable to handle this removal naturally. In the medical field, medical dialysis is simply referred to as dialysis. The removal of toxins, fluids, and…

An autoimmune disease resulting from an abnormality in neuromuscular transmission, characterized by a fluctuating degree of muscle weakness. Myasthenia gravis is a chronic autoimmune neuromuscular disorder (see illustration) characterized by varying…

A naturally occurring geologic body that may contain one or more metals. The metals may be present as native elements, or, more commonly, as oxides, sulfides, sulfates, silicates, or other compounds. The term ore is often used loosely to include…

A progressive degenerative disorder of the nervous system belonging to a group of conditions called motor system disorders. Parkinson's disease—more specifically known as idiopathic Parkinson's disease to distinguish it from the four Parkinson-plus…

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…

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…

A subdiscipline of paleobiology that investigates the processes of preservation and their influence on information in the fossil record. Taphonomy is the study of fossilization and considers the processes at work in the incorporation of living…

A rare genetic disorder in which there is a deficiency of hexosaminidase A (a lysosomal enzyme that hydrolyzes GM2 gangliosides). Tay-Sachs disease is a severe and progressive neurodegenerative disease. Because of the deficiency of hexosaminidase A…

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…

The process of forming a thrombus, which is a solid mass or plug in the living heart or vessels composed of the constituents of the blood. Thrombosis usually occurs in a diseased blood vessel, as a result of arteriosclerosis. The consequences of…

An aqueous solution of organic and inorganic substances—mostly waste products of metabolism—that collects in the kidneys and is removed from the body primarily through the urinary bladder and urinary tract. Urine is a liquid waste produced by the…

Diseases that are transmitted from animals to humans. Zoonoses (the singular form is zoonosis) are also termed zoonotic diseases. They are infectious diseases passed from animals (Fig. 1) to humans. Zoonotic agents are found among all major…

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…

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…

A viral disease of humans caused by the human immunodeficiency virus (HIV), which attacks and compromises the body's immune system. Individuals infected with the human immunodeficiency virus (HIV) proceed through a spectrum of stages that ultimately…

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…

Malfunctions of the paired adrenal glands. The paired adrenal glands are complex endocrine organs in proximity to the kidneys (see illustration), and disorders of the adrenal glands can have serious consequences. Under specific circumstances,…

A continent that straddles the Equator, extending between 37°N and 35°S. It is the second largest continent, exceeded only by Asia (see illustration). The political geography of the continent continues to evolve as the traditional states face…

A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often…

Mechanized systems of food and fiber production used in agriculture. The mechanized systems comprising agricultural machinery are utilized in numerous aspects of agriculture, including the initial tillage of the soil (Fig. 1), planting, cultural…

A sodium-rich plagioclase feldspar mineral whose composition extends over the range Ab100An0 to Ab90An10, where Ab (= albite) is NaAlSi3O8 and An (= anorthite) is CaAl2Si2O8 (see illustration). Albite occurs in crustal igneous rocks as a major…

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…

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 large aquatic reptile of the family Alligatoridae. Alligators belong to the family Alligatoridae in the reptilian order Crocodylia (Crocodilia). Common usage generally restricts the name alligator to the two extant species of the genus Alligator…

A small, tropical, evergreen tree (Pimenta dioica) and its dried, unripe fruit (berry). Allspice (Pimenta dioica; alternatively, P. officinalis) [see illustration] is a member of the myrtle family (Myrtaceae). Allspice, which is also known as…

Any of the plant growth forms characteristic of the upper reaches of mountain slopes. Alpine vegetation refers to the plants that grow in mountainous areas above the limit of subalpine trees and other subalpine vegetation. In such environments,…

An intergrading region on a mountain slope characterized by specific plant life forms or species composition, and determined by complex environmental gradients. Along an altitudinal transect of a mountain, there are sequential changes in the…

Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. The time necessary to transform resin…

A lithium aluminum phosphate mineral of basic formula LiAl(PO4)(F). The structure of amblygonite consists of phosphate (PO4) groups of tetrahedra and AlO6 groups of octahedra. Each PO4 tetrahedron is connected to an AlO6 octahedron. Corner-sharing…

Any protozoan moving by means of protoplasmic flow. In their entirety, ameboid protozoans (or ameboid protists) include naked amebas, those enclosed within a shell or test, and more highly developed representatives, such as heliozoians,…

The transparent purple to violet variety of the mineral quartz. Although quartz is perhaps the commonest gem mineral known, amethyst (see illustration) is rare in the deep colors that characterize fine quality. Amethyst is usually colored unevenly…

Any of the organic compounds possessing one or more basic amino groups and one or more acidic carboxyl groups. Of the more than 80 amino acids that have been found in living organisms, approximately 20 serve as the building blocks for proteins.…

A nesosilicate mineral, composition Al2SiO5, crystallizing in the orthorhombic system. Andalusite (Fig. 1) occurs commonly in large, nearly square prismatic crystals. The variety chiastolite has inclusions of dark-colored carbonaceous material…

A plagioclase feldspar with composition Ab70An30 to Ab50An50 (Ab = NaAlSi3O8; An = CaAl2Si2O8). Andesine (see illustration) occurs primarily in igneous rocks, often in a glassy matrix as small, chemically zoned, lathlike crystals known as…

A class of steroid hormones produced in the testes and adrenal cortex that act to regulate masculine secondary sexual characteristics. The major naturally occurring androgens, such as testosterone, are C-19 steroids (that is, they each contain 19…

A synthetic manufactured drug derived from the male sex hormone testosterone. Androgenic-anabolic steroids, often termed simply as anabolic steroids, are artificially produced testosterone variants. They can provide a number of medical benefits when…

Loss of sensation with or without loss of consciousness. Anesthesia is used to prevent pain during surgery and other medical procedures. There are several ways of producing anesthesia, with the choice dependent on the type of surgery and the medical…

A mineral with the chemical composition PbSO4. Anglesite occurs in white or gray, orthorhombic, tabular or prismatic crystals or compact masses (see illustration). It is a common secondary mineral, usually formed by the oxidation of galena. Fracture…

A mineral with the chemical composition CaSO4. Anhydrite (anhydrous calcium sulfate) occurs commonly in white and grayish granular masses, rarely in large, orthorhombic crystals (see illustration). Fracture is uneven and luster is pearly to…

The use of live nonhuman animals for scientific research and experimentation. Animal testing has been a cornerstone of biomedical research, having contributed greatly to the understanding of human biology, to the unraveling of the complex underlying…

The calcium-rich plagioclase feldspar with composition Ab0An100 to Ab10An90 (Ab = NaAlSi3O8; An = CaAl2Si2O8). Anorthite (see illustration) occurs in olivine-rich igneous rocks and rare volcanic ejecta (for example, at Mount Vesuvius, Italy, and…

An oxygen-depleted region in a marine environment. The dynamic steady state between oxygen supply and consumption determines the oceanic oxygen concentration. In regions where the rate of consumption equals or exceeds the rate of supply, seawater…

The coldest, windiest, and driest continent. The lowest temperature ever measured on Earth, −89.2°C (−128.5°F), was recorded at the Russian Antarctic station of Vostok in July 1983. Katabatic (cold, gravitational) winds with velocities up to 50 km/h…

A type of drug that inhibits the combination of histamine with histamine receptors. Antihistamine drugs are termed either H-1 or H-2 receptor antagonists, depending on which type of histamine receptor is involved. H-1 receptor antagonists are used…

One of the three living orders of the class Amphibia, encompassing the frogs and toads. Anura is the largest order of amphibians (class Amphibia), containing more than 7000 species of frogs (Fig. 1) and toads. Because members of the order Anura…

The application of ecological principles to the solution of human problems and the maintenance of a high quality of life. Applied ecology is a branch of ecology that applies concepts of ecology to address contemporary problems involving natural…

One of three naturally occurring mineral forms of calcium carbonate (CaCO3). The other forms (or polymorphs) are the abundant mineral calcite and the relatively rare mineral vaterite. Still other forms of calcium carbonate are known, but only as…

A natural order of the class Arachnida, commonly known as the spiders. Members of the order Araneae (also called Araneida)—the spiders (Fig. 1)—are widespread over most of the land areas of the world and are well adapted to many different habitats.…

A genus of fossil birds in the extinct order Archaeopterygiformes, characterized by flight feathers like those of modern birds. Archaeopteryx is generally accepted as the oldest known fossil bird. It represents a unique snapshot of evolution; most…

Arenite (rock composed of sand-size fragments) that contains a high proportion of feldspar in addition to quartz and other detrital minerals. Arkose is also known as feldspathic sandstone. Although there is no universal agreement, many geologists…

Any vegetable that is prevalent in Asian cuisines, but less well known in Western diets. Asian vegetables (Fig. 1), also referred to as Asian greens or oriental vegetables, are very important in Asian countries, but are not as popular in North…

Any treatment or procedure that includes the in-vitro handling of oocytes (immature ova or egg cells) and sperm or embryos for the purpose of establishing a pregnancy. Assisted reproductive technology (ART) is undertaken as a treatment of…

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 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 group of monoclinic calcic pyroxenes which have the general chemical formula (Ca,Mg,Fe)(Mg,Fe)-Si2O6, in which calcium is the dominant cation in the first cation position. Monoclinic pyroxene with substantial iron or magnesium in place of calcium…

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 basic carbonate of copper with the chemical formula Cu3(OH)2(CO3)2. Azurite is normally associated with copper ores and often occurs with malachite. Azurite is monoclinic. It may be massive or may occur in tabular, prismatic, or equant crystals…

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…

A highly contagious intestinal disease, also known as shigellosis, caused by rod-shaped bacteria of the genus Shigella. Bacillary dysentery, which also is referred to as shigellosis, is a significant intestinal infection. Rod-shaped bacteria of the…

Any of a group of drugs used for the suppression of anxiety, the induction of sleep, and the control of seizures. Barbiturates are sedative-hypnotic drugs, derived from barbituric acid, that act as central nervous system depressants (see…

A mineral with chemical composition BaSO4. It is isostructural with celestite, SrSO4, and anglesite, PbSO4. Barite is orthorhombic, space group Pnma, a = 0.887 nanometer, b = 0.545 nm, c = 0.714 nm. The illustration shows typical crystals. Barite…

The most common volcanic rock on Earth. Basalt is an igneous rock characterized by small grain size (less than about 5 mm) and approximately equal proportions of calcium-rich plagioclase feldspar and calcium-rich pyroxene, with less than about 20%…

The primary ore used to produce aluminum metal and alumina (Al2O3). Bauxite (see illustration) is a naturally occurring weathering product composed of one or more aluminum hydroxide minerals, such as gibbsite [Al(OH)3], boehmite [AlO(OH)], and…

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,…