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

When the new study’s team returned in 2020 and 2021, 43 percent of the sample sites had new, young trees with trunks that were at least two inches in diameter at chest height—something researchers hadn’t seen since the 1940s. —Sarah Kuta, Smithsonian Magazine, 28 July 2025

The planet killer that slammed into Earth 66 million years ago and led to the extinction of dinosaurs was estimated to be roughly 6 miles (about 10 kilometers) in diameter. —Ashley Strickland, CNN Money, 25 July 2025

Soiree Kawaii is a series of annual vinca (Catharanthus hybrid) flowers, between 1-2 inches in diameter, that grow into a one-foot mound and flower all summer. —Joshua Siskin, Oc Register, 25 July 2025

In more recent years, women also started competing in the standing block chop, where the log is vertical and 9 inches in diameter. —Maayan Silver, NPR, 25 July 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