Vacuum Definition: Vacuum refers to the given space of a gas below atmospheric pressure, i.e. the number of gas molecules in each cubic centimeter space is less than 250 billion given space. Vacuum is relative to atmospheric pressure. It is not that there is no matter in space. With the lowest pressure obtained by modern pumping methods, hundreds of molecules still exist in every cubic centimeter of space. Gas rarefaction is an objective measure of vacuum. The most direct physical measure is the number of gas molecules per unit volume. The smaller the molecular density of the gas, the lower the gas pressure and the higher the vacuum. However, due to historical reasons, measuring vacuum is usually indicated by pressure gauge. 1 Vacuum is commonly used as a unit of pressure in Pascal or Torr.