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

Plus, its 11-inch diameter provides plenty of room for seasonal decor, a vase, candles, and more. —Alicia Geigel, Southern Living, 27 Nov. 2025

Compared to Venus, the next brightest object that appears, the Moon is thirty times the diameter, takes up almost 1000 times the surface area, and appears about 1,000,000 times brighter than Venus. —Big Think, 26 Nov. 2025

Both solar prominences and filaments can extend for tens of thousands of miles, easily surpassing several times the diameter of Earth. —Kenna Hughes-Castleberry, Space.com, 26 Nov. 2025

To put that into context, the original, which was made of iron and wood and fitted with 100 old 25-watt light bulbs, measured 5 feet in diameter and weighed 700 pounds. —Rachel Cormack, Robb Report, 25 Nov. 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