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

If the Earth rotated about its axis every 24 hours, then your position in space would differ by the diameter of Earth — about 12,700 km (7,900 miles) — from the start of the night to the end of the night. —Big Think, 30 Oct. 2025

The pipes were miniature, with a diameter of only 18 millimeters and a wall thickness of just 3 millimeters. —Aman Tripathi, Interesting Engineering, 30 Oct. 2025

While there’s a fairly standard length for hair rollers, there can be significant variation in their diameter. —Bestreviews, Mercury News, 29 Oct. 2025

Once decorated, the tree, which is 45 feet in diameter, will glow with 50,000 LED lights. —Colson Thayer, PEOPLE, 27 Oct. 2025

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