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In geometry, a diameter (Greek words dia = through and metro = measure) of a circle is any straight line segment that passes through the centre of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle.

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. 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.

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 three 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, yA } .

In medical parlance the diameter of a lesion is the longest line segment whose endpoints are within the lesion.

Diameter symbol

Sign ⌀ from Autocad drawing

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 ⌀ 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 (⌀) with the empty set symbol (∅), similar to the uppercase Ø. Phi is sometimes used for diameter, although this seems to come from the fact that the symbols appear similar.

The diameter also refers to the approximate size of the corner of a frame of any given object to the nearest flat surface it represents.

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