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A torr is a unit of pressure defined as 1/760 of an atm. It was originally defined as, and is meant to approximate the pressure associated with 1 millimeter of the mercury column in a mercury barometer. Torr is not an SI unit but is based on one. Note: the word for the unit is torr (non-capitalized), but the symbol is Torr (capitalized), so 1/1000 of it can be written as a millitorr, with the associated symbol, mTorr.