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

Microscale printing advances Using the technique, the team fabricated microtubule arrays with individual tube diameters as small as 6 micrometers. — Neetika Walter, Interesting Engineering, 10 July 2026

The team observed that the critical stress level at which liquids fracture is proportional to their viscosity times the strain rate (how quickly they are being pulled or stretched apart and how the diameter of the liquid is changing). — Rohini Subrahmanyam, Quanta Magazine, 10 July 2026

Before purchasing one, check the diameter of your vacuum’s hose to ensure that the attachment will fit and is compatible with your model. — Caroline Lubinsky, Martha Stewart, 9 July 2026

The new photo was taken on July 2, according to China's Xinhua news outlet, and reveals the asteroid to be a small, asymmetrical rock measuring around 50–65 feet (16–20 meters) in diameter. — Brett Tingley, Space.com, 7 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