What is the unit of Vacuum ?

The unit of Vacuum is Torr.  Torr is named after Evangelista Toricelli, the inventor of the barometer. 

One Torr is equal to 1/760 of a standard atmosphere. 

One Torr is also defined as the pressure necessary to support a column of mercury which is 1 mm high at 0�C.  One Torr is equal to 1333.2 microbars.