noun di·am·e·ter \ dī-ˈa-mə-tər \
Updated on: 19 Feb 2018

Definition of diameter

1 mathematics : a chord (see 3chord 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


play \-ˈa-mə-trəl\ adjective

Examples of diameter in a Sentence

  1. The dotted line indicates the diameter of the circle.

  2. What is the diameter of the tree trunk?

  3. Dig a hole that's two feet deep and three feet in diameter.

Recent Examples of diameter from the Web

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'diameter.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

Origin and Etymology of diameter

Middle English diametre, from Middle French, from Latin diametros, from Greek, from dia- + metron measure — more at measure

Other Mathematics and Statistics Terms

DIAMETER Defined for English Language Learners



Definition of diameter for English Language Learners

  • : a straight line from one side of something (such as a circle) to the other side that passes through the center point

  • : the distance through the center of something from one side to the other

DIAMETER Defined for Kids


noun di·am·e·ter \ dī-ˈa-mə-tər \

Definition of diameter for Students

1 : a straight line that runs from one side of a figure and passes through the center
  • Measure the diameter of the circle.
2 : the distance through the center of an object from one side to the other : thickness
  • the diameter of a tree trunk

Word Root of diameter

The Greek word metron, meaning “measure,” gives us the root metr. Words from the Greek metron have something to do with measuring. A thermometer is an instrument that measures temperature. A diameter is the measurement across the center of a circle. The metric system is a system used for measuring size, weight, and volume. A perimeter is the measurement of the outside of a figure or shape.

Medical Dictionary


noun di·am·e·ter \ dī-ˈam-ət-ər \

medical Definition of diameter

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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