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

As trains traveling at high speeds displace enormous amounts of air, these tunnels require giant cross-sectional diameters to prevent uncomfortable passenger ear-popping and dangerous air-pressure shockwaves. — Mrigakshi Dixit, Interesting Engineering, 30 July 2026

Initial calculations suggested the roughly 340-meter-diameter rock had a one-in-37 chance of impacting Earth in 2029. — Andrew Jones, Scientific American, 30 July 2026

The torrent’s force moved boulders more than 20 feet in diameter and weighing more than 275 tons, equivalent to the heft of more than 150 Subaru Outbacks. — Elise Schmelzer, Denver Post, 27 July 2026

The large table is 25 inches in diameter and comes with a smaller 20-inch table that tucks stylishly underneath when not in use. — Better Homes & Gardens, 27 July 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