A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly 133. Medical definition of mm Hg: a unit of pressure equal to the pressure exerted by a column of mercury 1 millimeter high at 0°C and under the acceleration of. This page is all about abbreviation, acronym and meaning of the given term MMHG. What does mmHg mean? millimeters of Mercury (mmHg) is a unit used to measure pressure. MmHg definition: millimetre (s) of mercury (a unit of pressure equal to the pressure that can support a.