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 can't 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 760mm of mercury; hence 1/760th of an atmosphere, or 1 mm of mercury, has been a convenient measure of pressure for a long time, and is sometimes called a torr.
Because the standard atmosphere has been precisely defined (10th CGPM, 1954), and the standard atmosphere had previously been defined as 760 mmHg exactly, those two definitions are now combined to define the torr as exactly 101325/760 ≈ 133.3223684 pascals.
Although they are synonyms in practice, the torr and millimetre of mercury are very slightly different by virtue of their definitions in British Standard BS 2520 ([1], [2]).
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