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

Hopper balls range in diameter from 18 inches for toddlers and young kids to 29 inches for teenagers and adults. —Bestreviews, Chicago Tribune, 12 June 2026

Each stick measures approximately 7 inches in length and about 1 inch or less in diameter. —Michele Laufik, Martha Stewart, 12 June 2026

Storms could bring wind gusts up to 75 mph, destructive hail up to 2 inches in diameter and the possibility of tornadoes. —Francie Ebert, NBC news, 11 June 2026

Tiny silica beads about 10 microns in diameter are projected at the polymer at about 750 meters per second (over 1,600 miles per hour) with the amount of energy absorbed measured by calculating the change in particle velocity before and after the beads pass through. —Shirl Leigh june 10, New Atlas, 10 June 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