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

Each mirror is lightweight, 10 inches in diameter, and adds an interesting texture to walls. —Lauren Fischer, Better Homes & Gardens, 28 Jan. 2025

The destruction of the eagles’ nest is no small loss by many measures, as their nests can be 4 or 5 feet in diameter and weigh thousands of pounds, according to the U.S. Fish and Wildlife Service. —Olivia Lloyd, Charlotte Observer, 24 Jan. 2025

The case dimensions remain true to the original, measuring a modest 37mm in diameter. —Bhanu Chopra, Forbes, 22 Jan. 2025

The 251 brilliant diamonds totaling 28 carats is set with diamonds of three different diameters, the 60 largest of which weigh 20 carats and come with their own serial numbers. —Paige Reddinger, Robb Report, 21 Jan. 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

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