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

The Delta tunnel — 40 miles long and 36 feet in diameter — and Sites Reservoir would pump even more Sacramento River water, further harming salmon. —Scott Artis, Mercury News, 23 Sep. 2025

Their main burrow entrance is 10 to 12-inch diameter hole with burrow systems that are 30 feet in length. —Arricca Elin Sansone, Southern Living, 21 Sep. 2025

The circular piece has a 12-inch diameter that can pack away toys, small blankets, slippers, and more to declutter high-traffic areas. —Mariana Best, Better Homes & Gardens, 21 Sep. 2025

Saturn is roughly nine times wider than Earth, writes NASA, with a diameter of about 74,897 miles. —John Tufts, IndyStar, 21 Sep. 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