diameter

noun

di·​am·​e·​ter dī-ˈa-mə-tər 

1

mathematics : a chord (see chord entry 3 sense 2) passing through the center of a figure or body

2

mathematics : the length of a straight line through the center of an object or space

the diameter of a circle

dug a hole nearly four feet in diameter

3

physics : a unit of enlargement used with a number to indicate magnification by a lens or optical system

an object one millimeter wide magnified 40 diameters appears 40 millimeters wide

diametral

dī-ˈa-mə-trəl 

adjective

Examples of diameter in a Sentence

The dotted line indicates the diameter of the circle.

What is the diameter of the tree trunk?

Dig a hole that's two feet deep and three feet in diameter.

Recent Examples on the Web

The wheel, approximately a foot and a half in diameter, weighed 150 to 200 pounds. — Kori Rumore, Chicago Tribune, 28 Aug. 2026

The find from the Neolithic site Karahan Tepe measures about 28 inches tall and would have sat on a platform along the wall of a larger round building roughly 10 feet in diameter. — Francesca Aton, ARTnews.com, 27 Aug. 2026

Additionally, the average diameter of the cashmere fiber must not exceed 19 microns and the cashmere fibers in the wool product must contain no more than 3 percent (by weight) of cashmere fibers with average diameters that exceed 30 microns. — Angela Velasquez, Footwear News, 27 Aug. 2026

The Denver metro area, I-25 corridor and Eastern Plains are under a Level two severe threat with damaging winds, gusts of up to 60 mph and possible hail larger than two inches in diameter. — Alex Lehnert, CBS News, 26 Aug. 2026

Word History

Etymology

Middle English diametre, from Middle French, from Latin diametros, from Greek, from dia- + metron measure — more at measure

First Known Use

14th century, in the meaning defined at sense 1

Rhymes for diameter

• parameter
• pentameter
• semidiameter

Kids Definition

diameter

noun

di·​am·​e·​ter dī-ˈam-ət-ər 

1

: a straight line passing through the center of a figure or body

especially : a line segment through the center of a circle with its ends on the circle's circumference

2

: the length of a diameter

Medical Definition

diameter

noun

di·​am·​e·​ter dī-ˈam-ət-ər 

1

: a unit of magnification for an optical instrument equal to the number of times the linear dimensions of an object are apparently increased

a microscope magnifying 60 diameters

2

: one of the maximal breadths of a part of the body

the transverse diameter of the inlet of the pelvis