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

During the exercise, Sea Spear consistently detected, tracked and classified medium- and large-diameter UUVs. — Aditya Jadhav, Interesting Engineering, 24 July 2026

Photos and video from the area showed flames and smoke plumes rising over the ground station’s antenna array, anchored by a 230-foot-diameter (70-meter) radio dish and a collection of smaller 112-foot (34-meter) antennas. — Stephen Clark, ArsTechnica, 24 July 2026

The moon is huge—at least as massive as Jupiter, which is itself the largest planet in our solar system with a diameter 11 times that of Earth. — Jeffrey Kluger, Time, 22 July 2026

Rabbits can be kept out with a fence that’s 30 inches tall and made from hardware cloth or chicken wire with one-inch diameter holes to keep out the babies, too. — Arricca Elin Sansone, Southern Living, 21 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