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

Yet traditional metal alloys become difficult to manufacture and handle when their diameters shrink. —Aamir Khollam, Interesting Engineering, 6 Mar. 2026

This can be a big job; generally, trees more than 8 inches in diameter at the base are too heavy to remove by oneself. —Daniel Scott, Better Homes & Gardens, 5 Mar. 2026

Those with shorter fingers prefer shots with smaller diameters and those with longer fingers need larger ones. —Bestreviews, Chicago Tribune, 5 Mar. 2026

The stage is nearly equal in diameter to Orion and its service module, has been proven reliable on all of ULA's Vulcan missions to date, and doesn't need to be designed from scratch to support the needs of future Artemis launches. —Josh Dinner, Space.com, 4 Mar. 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