Millimeter of mercury
A millimeter of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high. It is now defined precisely as 13.5951 × 9.80665 = 133.322387415 pascals. It is denoted by the symbol "mmHg".
One millimeter of mercury is approximately 1 Torr, that is, 1/760 of standard atmospheric pressure. Even though the two units are not exactly equal, the difference (less than 0.000015%) is negligible for almost all practical uses.
History and definition 
Mercury manometers were the first practical accurate pressure gauges, and are still used in many scientific and technical fields. They displayed the pressure difference between two fluids as a vertical difference between the levels of mercury in two connected reservoirs. For that reason it became customary to measure pressure in "millimeters of mercury".
The actual pressure would be the height difference, times the density of mercury, times the local gravitational acceleration. Since the density of mercury depends on temperature, and the acceleration of gravity varies with location and altitude, at some point it was felt necessary to specify standard values for these two parameters. Eventually, the "millimeter of mercury" was defined as the pressure exerted at the base of a column 1 mm high of a fluid with density exactly 13.5951 g/cm3 when the acceleration of gravity is exactly 9.80665 m/s2.
The density 13.5951 g/cm3 chosen for this definition is the approximate density of mercury at 0 °C (32 °F), and 9.80665 m/s2 is the standard gravity.
In practice, of course, measurements are made using local values, which vary little enough at the Earth's surface. These assumptions limit both the validity and the precision of the mmHg as a unit of pressure.
Relation to the pascal 
According to the UK’s National Physical Laboratory (NPL), the assumption of fixed and exact (but ultimately incorrect) values of density and gravity will inherently limit knowledge of the relationship between the millimetre of mercury and the pascal. By contrast, the magnitude of pressure values expressed in the SI pressure unit, the pascal, can flex (albeit not by much) to take account of technological improvements in the underlying definitions of mass, length and time—the SI base quantities from which pressure is derived.
Relation to the torr 
The precision of modern transducers is sufficient to show the difference between the torr and the millimetre of mercury. However, instrument readings purported to be in "mmHg" may include large errors due to different definitions, different values of for gravity, or varying assumptions about the density and temperature. Misunderstandings about temperature assumptions alone can lead to errors of several tenths of a percent. 
Use in medicine and physiology 
In medicine, pressure is still generally measured in millimeters of mercury. These measurements are in general given relative to the current atmospheric pressure: for example, a blood pressure of 120 mmHg, when the current atmospheric pressure is 760 mmHg, means 880 mmHg relative to perfect vacuum.
Routine pressure measuremenets in medicine include:
- Blood pressure, measured with a sphygmomanometer
- Intraocular pressure, with a tonometer
- Cerebrospinal fluid pressure
- Intracranial pressure
- Intramuscular pressure (compartment syndrome)
- Central venous pressure
- Pulmonary artery catheterization
- Mechanical ventilation