*Not to be confused with Demeter.*

In geometry, the **diameter** of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the boundary of the circle. The diameters are the longest chords of the circle. The word "diameter" is derived from Greek *διάμετρος* (*diametros*), "diagonal of a circle", from *δια-* (*dia-*), "across, through" + *μέτρον* (*metron*), "a measure").

In more modern usage, the length of a diameter is also called the **diameter**. In this sense one speaks of *the* diameter rather than *a* diameter, because all diameters of a circle have the same length, this being twice the radius.

For a convex shape in the plane, the diameter is defined to be the largest distance that can be formed between two opposite parallel lines tangent to its boundary, and the *width* is defined to be the smallest such distance. Both quantities can be calculated efficiently using rotating calipers. For a curve of constant width such as the Reuleaux triangle, the width and diameter are the same because all such pairs of parallel tangent lines have the same distance. See also Tangent lines to circles. For a set of scattered points in the plane, the diameter of the points is the same as the diameter of their convex hull.

Read more about Diameter: Generalizations, Diameter Symbol

### Other articles related to "diameter":

... Fried described what has come to be known as Fried's seeing

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**diameter**is effectively a limiting aperture due to atmospheric turbulence, and is found either empirically or statistically ... The seeing

**diameter**limits optical resolution ...

**Diameter**Symbol

... The symbol or variable for

**diameter**, ⌀, is similar in size and design to ø, the Latin small letter o with stroke ... On an Apple Macintosh, the

**diameter**symbol can be entered via the character palette (this is opened by pressing ⌥ Opt⌘ CmdT in most applications), where it ... the symbol is achieved with the command

**diameter**which is part of the wasysym package ...

**Diameter**Credit-Control Application - Related Standards

... RFC 4005 -

**Diameter**Network Access Server Application ... RFC 4006 -

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