1. A unit of length equal to 0.001 meter. 2. A millimeter of mercury, abbreviated mm Hg, is a unit of pressure equivalent to the pressure exerted by a column of pure liquid mercury one mm high at 0°C under a standard gravity of 980.665 cm/s2; it is roughly equivalent to 1/760th of standard atmospheric pressure.

A millimetre (American spelling: millimeter, symbol mm) is one thousandth of a metre (the metre is the International System of Units (SI) base unit of length). The millimetre is part of a metric system. The corresponding unit of area is the square millimetre and the corresponding unit of volume is the cubic millimetre.