Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. Mathematics A straight line segment passing through the center of a figure, especially of a circle or sphere, and terminating at the periphery.
 n. Mathematics The length of such a segment.
 n. Thickness or width.
 n. A unit for measuring the magnifying power of a microscope lens or telescope, equal to the number of times an object's linear dimensions are apparently increased.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. Any straight line between two points on the circumference of a circle that passes through the centre/center of the circle.
 n. The length of such a line.
 n. The maximum distance between any two points in a metric space
 n. The maximum eccentricity over all vertices in a graph.
from the GNU version of the Collaborative International Dictionary of English
 n.
 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.
 n. A diametral plane.
 n. The length of a straight line through the center of an object from side to side; width; thickness.
 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.
from The Century Dictionary and Cyclopedia
 n. In geometry, a chord of a circle or a sphere which passes through its center; in general
 n. a chord of a conic cutting it at points tangents to which are parallel;
 n. a line intersecting a quadric surface at points where the tangent planes are parallel.
 n. 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.
 n. 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 WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. the length of a straight line passing through the center of a circle and connecting two points on the circumference
 n. 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
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.

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

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.

If you want to make sliceandbake cookies, shape each half into a chubby sausage (the diameter is up to you — I usually like cookies that are about 2 inches in diameter) and wrap in plastic.

A pit dug into the earth and measuring about eight feet deep by twelve feet in diameter is preheated for a couple of days with thick smoldering logs, on top of which are then placed river rocks.

The hollow sphere, ten feet in diameter, is turned by water power; it has a map of the constellations on the interior and a map of the world on the outside.

Each fiber, just onemilimeter in diameter, is made out of a photoconductive glass core, electrodes, and a transparent sheathing.
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