diameter

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

• noun A straight line segment passing through the center of a figure, especially of a circle or sphere, and terminating at the periphery.
• noun The length of such a segment.
• noun Thickness or width.
• noun A unit of magnification equal to the number of times an object's linear dimensions is increased by the magnifying apparatus.

from The Century Dictionary.

• noun The diameter (see def. 2) of the object observed, taken as a convenient measure of linear magnification used in micros-copy and in telescopic work.
• noun In geometry, a chord of a circle or a sphere which passes through its center; in general
• noun a chord of a conic cutting it at points tangents to which are parallel;
• noun a line intersecting a quadric surface at points where the tangent planes are parallel.
• noun The length of a diameter; the thickness of a cylindrical or spherical body as measured, in the former case on a diameter of a cross-section made perpendicular to the axis, and in the latter on a line passing through the center: as, a tree two feet in diameter; a ball three inches in diameter.

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

• noun 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.
• noun A diametral plane.
• noun The length of a straight line through the center of an object from side to side; width; thickness.
• noun (Arch.) 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.
• noun See under Conjugate.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geometry Any straight line between two points on the circumference of a circle that passes through the centre/center of the circle.
• noun geometry The length of such a line.
• noun geometry The maximum distance between any two points in a metric space
• noun graph theory The maximum eccentricity over all vertices in a graph.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the length of a straight line passing through the center of a circle and connecting two points on the circumference
• noun a straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface)