# Diameter

*For the authentication, authorisation, and accounting protocol, see DIAMETER.*

In geometry, a **diameter** (Greek words *diairo* = divide and *metro* = measure) of a circle is any straight line segment that passes through the center and whose endpoints are on the circular boundary, or, in more modern usage, the length of such a line segment. When using the word in the more modern sense, one speaks of *the* diameter rather than *a* diameter, because all diameters of a circle have the same length. This length is twice the radius. The diameter of a circle is also the longest chord that the circle has.

The **diameter** of a connected graph is the distance between the two vertices which are furthest from each other. The distance between two vertices *a* and *b* is the length of the shortest path connecting them (for the length of a path, see Graph theory).

The two definitions given above are special cases of a more general definition. The **diameter** of a subset of a metric space is the least upper bound of the distances between pairs of points in the subset. So, if *A* is the subset, the diameter is

- sup { d(
*x*,*y*) |*x*,*y*in*A*} .

## Diameter symbol

The symbol or variable for diameter is similar in size and design to ø, the lowercase letter o with stroke. Unicode provides character number 8960 (hexadecimal 2300) for the symbol, which can be encoded in HTML webpages as `⌀` or `⌀`. Proper display of this character, however, is unlikely in most situations, as most fonts do not have it included. *(Your browser displays ⌀ and ⌀ in the current font.)* In most situations the letter ø is acceptable, obtained in Microsoft Windows by holding the `[Alt]` key down while entering` 0 2 4 8 ` on the numeric keypad.

It is important not to confuse a diameter symbol (ø) with the empty set symbol, similar to the uppercase Ø. Diameter is also sometimes called phi (pronounced the same as "fee"), although this seems to come from the fact that Ø and ø look like Φ and φ, the letter phi in the Greek alphabet.

See also: angular diameter, hydraulic diameterbg:Диаметър ca:Diàmetre da:Diameter de:Durchmesser es:Diámetro eo:Diametro fr:Diamètre is:Þvermál he:קוטר li:Diameter nl:Diameter ja:径 no:Diameter nn:Diameter pl:Średnica pt:Diâmetro ru:Диаметр sl:Premer sr:Пречник sv:Diameter ta:விட்டம் uk:Діаметр zh:直径