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

Totems and totems with flags, less than 7 feet tall, and less than 1 inch in diameter. —Ed Masley, AZCentral.com, 19 Feb. 2026

River conservationists on Wednesday urged state regulators to reassess how aging hydropower infrastructure is operated and maintained — and, in some cases, whether certain facilities should remain in place — following a 14-foot diameter, high-pressure water pipe ruptured last week. —Chaewon Chung, Sacbee.com, 19 Feb. 2026

There will also be an observation deck (currently planned to be the world’s tallest) and sky terrace, a Four Seasons luxury hotel, and a 98-foot-diameter outdoor balcony, which was originally designed as a helipad. —Katherine McLaughlin, Architectural Digest, 18 Feb. 2026

Because the moon’s diameter appears smaller than that of the sun, our star looks to us like a fiery halo of light in the sky, hence the eclipse’s nickname. —Jackie Flynn Mogensen, Scientific American, 17 Feb. 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

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