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 small asteroid, which is little more than 3 miles in diameter, approaches so close to the sun that it was named for the son of the sun-god Helios, who in Greek mythology lost control of his father's chariot and set the Earth ablaze. —Eric Lagatta, USA Today, 10 Dec. 2025

Neutron will stand 141 feet (43 meters) tall with a 23-foot (7 m) diameter, and is expected to deliver up to 28,700 pounds (13,000 kilograms) of payload to low Earth orbit. —Josh Dinner, Space.com, 10 Dec. 2025

Children may be particularly vulnerable because of their smaller airway diameters and greater deposition volume per body weight. —Daniella Gray, MSNBC Newsweek, 9 Dec. 2025

There are currently more than 25,000 pieces of space junk larger than 4 inches in diameter circling the Earth, according to documentation from NASA. —Maggie Koerth, CNN Money, 8 Dec. 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