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

However, the value for the sun’s diameter appears to be wrong, leaving eclipse maps — projections of the moon’s shadow — slightly askew. — Jamie Carter, Forbes.com, 7 Aug. 2026

The Metropolitan District Commission is seeking to install a new 30-inch diameter water transmission main through Windsor, Hartford, South Windsor and East Hartford and beneath both the Connecticut River and Podunk River. — Sean Krofssik, Hartford Courant, 6 Aug. 2026

Engineers plan to drill larger-diameter wells and install additional prototype equipment before beginning a full-scale reactor demonstration. — Aamir Khollam, Interesting Engineering, 6 Aug. 2026

These mRNA-lipid nanoparticles, or LNPs, are tiny spheres about 100 nanometers in diameter that protect mRNA from degradation and facilitate its delivery into target cells. — Li Li, The Conversation, 6 Aug. 2026

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
• semidiameter

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