Noah Webster's New International Dictionary of the English Language

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.