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

Rounded flower clusters about an inch in diameter usually consist of light yellow blooms in the center with orange, red, pink, peach, yellow, or white blooms surrounding them, though some selections offer solid colors. —Steve Bender, Southern Living, 12 May 2026

All modes of severe weather will be likely, but the main threat includes a significant risk of hail up to 2 inches in diameter and winds up to category 1 hurricane strength. —Lauren Bostwick, CBS News, 9 May 2026

The figure also remains near the 50-inch diameter of the F119-PW-100 powering the Lockheed Martin F-22 Raptor, reported Army Recognition. —Prabhat Ranjan Mishra, Interesting Engineering, 8 May 2026

Its diameter is about 860 miles (the distance from Salt Lake City to Topeka, Kansas) and has a volume of about 332,500,000 cubic miles. —Kurt Snibbe, Oc Register, 8 May 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