Millimeters of mercury (mmHg) is a unit of pressure measurement. It is defined as the pressure exerted by a column of mercury that is 1 millimeter high at standard gravity. This unit is commonly used in various scientific fields, including medicine and meteorology, to express blood pressure and atmospheric pressure, respectively. In medicine, mmHg is often used to measure blood pressure, with normal readings typically around 120/80 mmHg. In meteorology, atmospheric pressure is measured in mmHg to help predict weather patterns. The use of mercury in these measurements dates back to the invention of the barometer by Evangelista Torricelli in the 17th century.