It's good to know where the diameter of a circular object is if you have to split a cookie between two 5-year-old kids. Think of a round chocolate chip cookie. The diameter of the cookie is a line that divides the cookie into two equal halves, through the center of the cookie.

The diameter of a circle or sphere is a line that passes through the center and starts and ends and two opposite points on its perimeter or edge. The diameter is also the length of that line. In math class, teachers often ask you to use the diameter of a circle to find its area. This is a good skill to have if you are trying to figure out whether the 14 inch or 16 inch pizza is a better deal.

Definitions of diameter
  1. noun
    the length of a straight line passing through the center of a circle and connecting two points on the circumference
    synonyms: diam
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    bore, caliber, calibre, gauge
    diameter of a tube or gun barrel
    the space between the projectile of a smoothbore gun and the surface of the bore of the gun
    type of:
    the linear extent in space from one end to the other; the longest dimension of something that is fixed in place
  2. noun
    a straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface)
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    type of:
    straight line
    a line traced by a point traveling in a constant direction; a line of zero curvature
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