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

This building has a diameter of 80 feet and a height of 100 feet. —Aman Tripathi, Interesting Engineering, 9 Apr. 2026

The asteroid, designated 2026 GD, was discovered on Monday (April 6) and is estimated to be 16 meters (54 feet) in diameter. —Anthony Wood, Space.com, 9 Apr. 2026

At 516 feet in width, the unusual structure is roughly 22,000 times smaller than the actual Moon, which with a diameter of 2,159 miles could roughly fit the contiguous United States across one of its halves from east to west. —Victor Tangermann, Futurism, 9 Apr. 2026

Some severe thunderstorms can produce hail larger than softballs — or 11 to 12 inches in diameter — and winds of more than 100 mph. —Sacbee.com, 8 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