Diameter

The word "diameter" is derived from Ancient Greek: διάμετρος (diametros), "diameter of a circle", from διά (dia), "across, through" and μέτρον (metron), "measure".

With straightedge and compass, a diameter of a given circle can be constructed as the perpendicular bisector of an arbitrary chord.

Drawing two diameters in this way can be used to locate the center of a circle, as their crossing point.

[4] The symbol has a code point in Unicode at U+2300 ⌀ DIAMETER SIGN, in the Miscellaneous Technical set.

It should not be confused with several other characters (such as U+00D8 Ø LATIN CAPITAL LETTER O WITH STROKE or U+2205 ∅ EMPTY SET) that resemble it but have unrelated meanings.

However, they are special cases of a more general definition that is valid for any kind of

In this context, a diameter is any chord which passes through the conic's centre.

[7] Half of any such diameter may be called a semidiameter, although this term is most often a synonym for the radius of a circle or sphere.

Jung's theorem provides more general inequalities relating the diameter to the radius.