The torr is a non SI unit of pressure, named after Evangelista Torricelli. Its symbol is Torr.
One way of defining pressure is in terms of the height of a column of fluid that may be supported by that pressure; or the height of a column of fluid that exerts that pressure at its base. Although a manometer may use any fluid in principle, common fluids like water give heights that cannot be contained in a normal room. A water column needs to be of the order of 10 metres to give atmospheric pressure. Therefore, a very dense fluid is required - mercury. Normal atmospheric pressure can support around 760 mm of mercury; hence 1/760th of an atmosphere, or 1 mm of mercury (mmHg), has been a convenient measure of pressure for a long time, and is sometimes also called a torr.
Because the standard atmosphere is now a defined quantity in the SI system of units, the torr is hence defined as exactly 101325 / 760 (or 2533.125 / 19) ≈ 133.3223684 pascals. Although the torr is still in common use in low-pressure engineering, the pascal is now the recommended unit of pressure.
This unit, usually under the millimetre of mercury name remains the most common unit for the measurement of blood pressure in much of the world.
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