Atmospheric Pressure: It is the weigth of the air per unit surface area of earth and Denoted by P(atm).
Atmospheric pressure is inversely proportional to height and its standard value is 760 mm of Hg.
Absolute Pressure: If pressure is measured relative to absolute zero, it is called absolute pressure.
Gauge pressure: If pressure is measured relative to atmospheric pressure as a base, it is called gauge pressure.
Gauge pressure are positive if they are above that of the atmosphere and negative if they are vacuum.
Vacuum pressure or negative gauge pressure: If the pressure is below that of the atmosphere, it is designated as a vacuum pressure and its gauge value is amount by which it is below that of the atmospheric pressure.