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

These fibers are extremely thin, about 1/100 the diameter of human hair and 1/20 the diameter of a strand of silk. —Nafeesah Allen, Better Homes & Gardens, 17 Oct. 2025

Tests were conducted on a flexible vine robot ranging from 3 to 7 millimeters in diameter and 25 centimeters in length. —Neetika Walter, Interesting Engineering, 16 Oct. 2025

It is estimated that a single large-area satellite, one 50 meters in diameter, would encounter over 100 micrometeoroid strikes each year in low-Earth orbit. —Big Think, 15 Oct. 2025

Oaks, dogwoods, and maples over 12 inches in diameter are trees most often affected by slime flux. —Mary Marlowe Leverette, Southern Living, 15 Oct. 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