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

Some storms could produce hail larger than 3 inches in diameter, especially with stronger rotating cells. —Brandi D. Addison, USA Today, 28 Apr. 2026

The recent excavation revealed a massive basin with a diameter of 115 feet, which was once filled with water from the Nile River. —Andrea Margolis, FOXNews.com, 28 Apr. 2026

Branches can be between three and eight feet long and no greater than six inches in diameter. —Michelle Mullins, Chicago Tribune, 27 Apr. 2026

At the microscale, the team fabricated porous, sponge-like particles roughly 2 micrometres in diameter, significantly thinner than a human hair, and tuned their internal chemistry to maintain long-term stability in aqueous environments. —Bojan Stojkovski, Interesting Engineering, 26 Apr. 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