Diameter

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 } .

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.

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