axis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A straight line about which a body or geometric object rotates or may be conceived to rotate.
• noun An unlimited line, half-line, or line segment serving to orient a space or a geometric object, especially a line about which the object is symmetric.
• noun A reference line from which distances or angles are measured in a coordinate system.
• noun A center line to which parts of a structure or body may be referred.
• noun An imaginary line to which elements of a work of art, such as a picture, are referred for measurement or symmetry.
• noun The second cervical vertebra on which the head turns.
• noun Any of various central structures, such as the spinal column, or standard abstract lines used as a positional referent.
• noun Botany The main stem or central part about which organs or plant parts such as branches are arranged.
• noun One of three mutually perpendicular lines that define the orientation of an aircraft, with one being along its direction of travel and the other two being perpendicular to the direction of travel.
• noun A line through the optical center of a lens that is perpendicular to both its surfaces.
• noun One of three or four imaginary lines used to define the faces of a crystal and the position of its atoms.
• noun An alliance of powers, such as nations, to promote mutual interests and policies.
• noun The alliance of Germany and Italy in 1936, later including Japan and other nations, that opposed the Allies in World War II.

from The Century Dictionary.

• noun 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.
• noun 2. [capitalized] [NL.] A genus of such deer.
• noun In architecture, one of several lines, either imagined or laid down in the plans, etc., which pass through the middle of an important feature.
• noun 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.)
• noun 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).
• noun The axle of a wheel; the cylindrical portion of any mechanical piece intended to turn in bearings: as, the axis of a transit instrument.
• noun 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.
• noun 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).
• noun The odontoid process of the axis.
• noun The entire vertebral column.
• noun The central or axial nervous system of a vertebrate: as, the cerebrospinal axis.
• noun The columella or modiolus of the cochlea.
• noun A short thick artery which immediately divides into several branches: as, the celiac axis; the thyroid axis.
• noun The axis-cylinder of a nerve.
• noun Same as axon.
• noun 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.
• noun Any line with reference to which the physical properties of a body, especially its elasticity, are symmetrical.
• noun 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.
• noun In conchology, the imaginary line or space around which the whorls of a spiral shell turn.
• noun 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.
• noun 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.
• noun In analytical geometry, any fixed line of reference used to determine the position of a point or series of points (line, surface) in space.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) The spotted deer (Cervus axis or Axis maculata) of India, where it is called hog deer and parrah (Moorish name).
• noun A straight line, real or imaginary, passing through a body, on which it revolves, or may be supposed to revolve; a line passing through a body or system around which the parts are symmetrically arranged.
• noun (Math.) A straight line with respect to which the different parts of a magnitude are symmetrically arranged
• noun (Bot.) The stem; the central part, or longitudinal support, on which organs or parts are arranged; the central line of any body.
• noun The second vertebra of the neck, or vertebra dentata.
• noun Also used of the body only of the vertebra, which is prolonged anteriorly within the foramen of the first vertebra or atlas, so as to form the odontoid process or peg which serves as a pivot for the atlas and head to turn upon.
• noun (Crystallog.) One of several imaginary lines, assumed in describing the position of the planes by which a crystal is bounded.
• noun (Fine Arts) The primary or secondary central line of any design.
• noun (Geol.) a line or ridge from which the strata slope downward on the two opposite sides.
• noun a line from which the strata slope upward in opposite directions, so as to form a valley.
• noun (Anat.) the neuraxis or essential, central substance of a nerve fiber; -- called also axis band, axial fiber, and cylinder axis.