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noun di·am·e·ter \dī-ˈa-mə-tər\

Simple Definition of diameter

  • : 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

Source: Merriam-Webster's Learner's Dictionary

Full Definition of diameter

  1. 1 :  a chord passing through the center of a figure or body

  2. 2 :  the length of a straight line through the center of an object

  3. 3 :  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 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.

Origin of diameter

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

First Known Use: 14th century

Other Mathematics and Statistics Terms

DIAMETER Defined for Kids


noun di·am·e·ter \dī-ˈa-mə-tər\

Definition of diameter for Students

  1. 1 :  a straight line that runs from one side of a figure and passes through the center <Measure the diameter of the circle.>

  2. 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. 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. 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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