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
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
Each unit is around 250 nanometers (nm) in diameter and boasts a large surface area, as well as strong electrochemical capacity.
—
Georgina Jedikovska, Interesting Engineering, 5 Feb. 2026
The gas giant is about 11 times wider than Earth alone, with a diameter around its equator of 88,846 miles.
—Eric Lagatta, USA Today, 5 Feb. 2026
The resolution of a telescope (or any instrument, including your eye) is dependent on how many wavelengths of light fit across the diameter of your telescope (or eye), and so the larger your telescope is, the better its resolution is.
—Big Think, 4 Feb. 2026
The button, the middle of house, is on the intersection of those two lines, and it is surrounded by three concentric rings with diameters of four, eight and 12 feet.
—Matt Slater, New York Times, 4 Feb. 2026
See All Example Sentences for diameter
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Georgina Jedikovska, Interesting Engineering, 5 Feb. 2026
The gas giant is about 11 times wider than Earth alone, with a diameter around its equator of 88,846 miles.
—Eric Lagatta, USA Today, 5 Feb. 2026
The resolution of a telescope (or any instrument, including your eye) is dependent on how many wavelengths of light fit across the diameter of your telescope (or eye), and so the larger your telescope is, the better its resolution is.
—Big Think, 4 Feb. 2026
The button, the middle of house, is on the intersection of those two lines, and it is surrounded by three concentric rings with diameters of four, eight and 12 feet.
—Matt Slater, New York Times, 4 Feb. 2026
See All Example Sentences for diameter
Cite this Entry
“Diameter.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/diameter. Accessed 8 Feb. 2026.
Copy CitationKids 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
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