Definitions
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 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 crosssection 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.
 noun The diameter (see def. 2) of the object observed, taken as a convenient measure of linear magnification used in microscopy and in telescopic work.
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/ShareAlike License.
 noun geometry Any
straight line between two points on thecircumference of acircle that passes through thecentre /center of the circle.  noun geometry The length of such a line.
 noun geometry The maximum
distance between any two points in ametric space  noun graph theory The maximum
eccentricity over allvertices 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)
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
Examples

How many millimeters in diameter is the watch case?

Local lore has long maintained the tree, whose massive partially hollowed trunk measures 7 feet in diameter, is the very one featured in the biblical tale of Jesus and Zacchaeus, the tax collector of short stature who, according to the Gospel of Luke, climbed the tree to get a better look at Jesus.

Local lore has long maintained the tree, whose massive partially hollowed trunk measures 7 feet in diameter, is the very one featured in the biblical tale of Jesus and Zacchaeus, the tax collector of short stature who, according to the Gospel of Luke, climbed the tree to get a better look at Jesus.

Then, a line of tiny holes, less than onemillionth of a meter in diameter, is punched into the metal, spaced five micrometers apart.

Local lore has long maintained the tree, whose massive partially hollowed trunk measures 7 feet in diameter, is the very one featured in the biblical tale of Jesus and Zacchaeus, the tax collector of short stature who, according to the Gospel of Luke, climbed the tree to get a better look at Jesus.

Local lore has long maintained the tree, whose massive partially hollowed trunk measures 7 feet in diameter, is the very one featured in the biblical tale of Jesus and Zacchaeus, the tax collector of short stature who, according to the Gospel of Luke, climbed the tree to get a better look at Jesus.

Then, a line of tiny holes, less than onemillionth of a meter in diameter, is punched into the metal, spaced five micrometers apart.

The formation, measuring 150ft in diameter, is apparently a coded image representing the first 10 digits, 3.141592654, of pi.
Most Complex Crop Circle Ever Discovered in British Fields  Disinformation

The seat, which measures 47 cm in diameter, is upholstered with 50 mm of soft foam, and the conical back is clad with 20 mm of the same soft foam.

Machon 1 – The large silverdomed reactor containment vessel, nearly 20 meters [about 60 feet] in diameter, is visible from a nearby highway.
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