A measurement of pressure.
The main use of this unit I have seen is medical, for determining blood pressure. mmHg expands to mm Hg, and from that to Millimeters of Mercury (Hg is the chemical symbol for mercury).
I am told by Professor Pi that it is also known as the Torr. It is the pressure in a column of mercury 1mm high.
An example would be '120 mmHg' | 
