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 specifications of the Fission Surface Power Project called for a system that weighs just 6 tons, can fit into a cylinder 13 feet in diameter and 20 feet long, and can work for 10 years, self-regulated, with no maintenance or refuelling required. —Christopher Helman, Forbes.com, 7 Aug. 2025

Harvest when the bulb is about the size of a tennis ball, 2 to 3 inches in diameter. —Karen Brewer Grossman, Southern Living, 7 Aug. 2025

In its medusa form, the jellyfish grows to up to 25mm in diameter, about the size of a nickel coin. —Keith Matheny, Freep.com, 6 Aug. 2025

Developed entirely in-house, and built in a partnership with China’s CATL, the nickel-rich cells feature a 46-millimeter diameter and come in two heights, either 95 or 120 mm, for greater flexibility across different models. —IEEE Spectrum, 4 Aug. 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