Millimetre

The millimetre (American spelling: millimeter, symbol mm) is a unit of length in the metric system, equal to one thousandth of a metre, which is the current SI base unit of length.

The corresponding unit of area is the square millimetre and the corresponding unit of volume is the cubic millimetre.

Unicode symbols
For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for:
 * millimetre (㎜) - code 339C
 * square millimetre (㎟) - code 339F
 * cubic millimetre (㎣) - code 33A3

They are useful only with East Asian fixed-width CJK fonts, because they are equal in size to one Chinese character.