noun
 Geometry.
 a straight line passing through the center of a circle or sphere and meeting the circumference or surface at each end.
 a straight line passing from side to side of any figure or body, through its center.
 the length of such a line.
 the width of a circular or cylindrical object.
noun

 a straight line connecting the centre of a geometric figure, esp a circle or sphere, with two points on the perimeter or surface
 the length of such a line
 the thickness of something, esp with circular cross section
late 14c., from Old French diametre, from Latin diametrus, from Greek diametros (gramme) “diagonal of a circle,” from dia “across, through” (see dia) + metron “a measure” (see meter (n.2)).
n.
 A straight line connecting two opposite points on the surface of a spherical or cylindrical body, or at the boundary of an opening or foramen, passing through the center of such body or opening.
 The distance measured along such a line.
 A straight line segment that passes through the center of a circle or sphere from one side to the other.
 The length of such a line segment.
A straight line passing through the center of a figure, especially a circle or sphere, and joining two opposite points on its circumference.