Vacuum [Latin word "empty"] is well defined in several documents. According to the definition of the American Vacuum Society (1958) the term "vacuum" refers to a given space filled with gas at pressures below atmospheric, i.e. having a density of molecules less than about 2.5x1019 molecules/cm3. In other word, a vacuum is the state of a gas where the density of the particles is lower than atmospheric pressure at the earth's surface. In general the term "vacuum" includes nowadays about 19 orders of magnitude of pressures (or densities) below that corresponding to the standard atmosphere.

The vacuum can be divided into two catagories, artificial and natural vacua. 1. Artificial Vacuum: On the earth vacuum is normally achieved by pumping on a vessel, the degree of vacuum increasing as the pressure exerted by the residual gas decreases below atmosphere. The degree of vacuum is classified by measuring a system's absolute pressure. Traditional limits of vacuum ranges are rather arbitrary. Nonetheless, low, medium, high, and ultrahigh vacuum can be specified corresponding to regions oflower and lower pressures. The concept of vacuum is normally undersood in terms of molecular density, mean free path, and the time constant to form a monolayer

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