Torr is a measurement of hard vacuum.

Atmospheric pressure at seal level is 760 mm of mercury. Any thing less than 760 mm is described as a vacuum.

One Torr is defined as one millimeter of mercury. The term is used when seal or pump people are describing hard vacuum.

Mechanical seals can seal hard vacuums to about 0.001 Torr. Below this point, the air in the elastomer (O-ring) will out-gas causing the O-ring to shrink and leak.