n. The motionless, or relatively motionless, imaginary line about which a rotating body, such as the earth, turns: specifically called in this sense the axis of revolution or rotation (which see, below).n. The axle of a wheel; the cylindrical portion of any mechanical piece intended to turn in bearings: as, the axis of a transit instrument.n. In anatomy: The second cervical vertebra: so called because the atlas turns upon it as about a pivot or axis, bearing the head with it.n. It is usually distinguished from the other vertebræ by having an odontoid or tooth-like process, furnishing the pivot about which the atlas turns: hence called the toothed vertebra (vertebra dentata), or the odontoid vertebra (vertebra odontoidea).n. The odontoid process of the axis.n. The entire vertebral column.n. The central or axial nervous system of a vertebrate: as, the cerebrospinal axis.n. The columella or modiolus of the cochlea.n. A short thick artery which immediately divides into several branches: as, the celiac axis; the thyroid axis.n. The axis-cylinder of a nerve.n. Same as axon.n. The central line of a solid of revolution; the central line of any symmetrical, or nearly symmetrical, body: as, the axis of a cylinder, of the eye, etc.n. Any line with reference to which the physical properties of a body, especially its elasticity, are symmetrical.n. In Trilobita, specifically, the tergum; the median convex portion of a thoracic somite, intervening between the pleura or flattened lateral portions of the thorax. See cut under Trilobita.n. In conchology, the imaginary line or space around which the whorls of a spiral shell turn.n. In botany, the stem; the central part or longitudinal support on which organs or parts are arranged. The root has sometimes been called the descending axis.n. In geography and geology, the central or dominating region of a mountain-chain, or the line which follows the crest of a range and thus indicates the position of the most conspicuous portion of the uplift.n. In analytical geometry, any fixed line of reference used to determine the position of a point or series of points (line, surface) in space.n. A kind of East Indian deer, Cervus axis, of which there are several varieties, perhaps species. The body is spotted with white. Also called axis-deer, spotted deer, and hog-deer.n. 2. [capitalized] [NL.] A genus of such deer.n. In architecture, one of several lines, either imagined or laid down in the plans, etc., which pass through the middle of an important feature.n. of a lens, any straight line through the center of the lens which does not coincide with the radii of curvature. (Used only in the approximate theory of lenses, in which the thickness is ignored.)