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

To convert the solid preforms into fiber, workers load them into multi-story draw towers where they are heated and pulled down to 125 microns in diameter — roughly the width of a human hair. — Charlotte Observer, 15 Sep. 2026

However, at infrared wavelengths the brightness of an object is dependent mostly on its size, allowing accurate determinations of the diameters of the 27 TNOs. — Keith Cooper, Space.com, 14 Sep. 2026

Scientists from the German Aerospace Center’s Institute of Space Research reported last week that Mercury’s shrinkage may be 30% greater than previously believed — a loss in diameter of as much as 14 miles. — Marcia Dunn, Los Angeles Times, 13 Sep. 2026

Scientists from the German Aerospace Center's Institute of Space Research reported Thursday that Mercury's shrinkage may be 30% greater than previously believed -- a loss in diameter of as much as 14 miles. — Arkansas Online, 11 Sep. 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