1. (n.) Any right line passing through the center of a figure or body, as a circle, conic section, sphere, cube, etc., and terminated by the opposite boundaries; a straight line which bisects a system of parallel chords drawn in a curve.

2. (n.) A diametral plane.

3. (n.) The length of a straight line through the center of an object from side to side; width; thickness; as, the diameter of a tree or rock.

4. (n.) The distance through the lower part of the shaft of a column, used as a standard measure for all parts of the order. See Module.

air line amplitude area axis beeline bigness bisector body bore boundary breadth bulk caliber center chord core coverage depth diagonal diaphragm dimension dimensions direct line directrix divider edge equator expanse expansion extension extent gauge girth great-circle course greatness halfway mark heart height interior kernel largeness length line of demarcation magnitude mass mean measure measurement median middle midmost midriff midst normal nucleus partition perpendicular proportion proportions radius radius vector range reach right line scale scope secant segment semidiameter shortcut side size spread straight straight course straight line straight stretch straightaway streamline tangent thick thick of things transversal vector volume waist waistline width zone