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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 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…

The cross section of a body that is placed in an airstream to produce a useful aerodynamic force in the most efficient manner possible. The cross sections of wings, helicopter rotor blades (Fig. 1), propeller blades, windmill blades, compressor and…

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…

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 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…

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…

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…

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…

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 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…

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…

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…

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…

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.…

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 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…

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…

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 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%…

Bats are members of the order Chiroptera, which is the second-largest order of living mammals. The geographic distribution of these flying mammals is tremendous as well, ranging from the limit of trees in the Northern Hemisphere to the southern tips…