The torr (symbol: Torr)'s a non-SI unit of pressure defined as of a standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimeter of mercury. Note that the symbol's spelled exactly the same as the unit, but the symbol's capitalized, as's customary in metric units derived from names. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644. History Torricelli attracted considerable atte… (More on Torr)