In geometry, the diameter of a circle is the length of a straight line segment that passes from a point on the circle to the opposite point (and therefore passes through the centre of the circle). This length is twice the radius. The line segment itself is also called a diameter.

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 } .

A version of the
diameter symbol.
The symbol or variable for diameter is similar in size and design to , 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 is acceptable, obtained in Windows by holding the [Alt] key down while entering 0 2 4 8 on the numeric keypad. A magnified version of a diameter symbol is shown at right.

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