Saturation Vapor Pressure
Imagine a small, closed, box half filled with water. To start with air will be completely dry, however water will start to evaporate and the number of water molecules in the box, and therefore the water vapor pressure, will increase. If this pressure continues to increase, the rate at which water molecules condense from the vapor phase back to the liquid phase will also increase.
If the rate of condensation is less than the rate of evaporation, the box is said to be unsaturated at temperature T. When the water vapor pressure in th box increases to the point where the rate of condensation equals the rate of evaporation, the air is said to be saturated with respect to a plane surface of pure water at temperature T. The pressure that is then excerted by the water vapor is then called the saturation vapor pressure.
More simply put, the saturation vapor pressure is the the critical vapour pressure at which the number of molecules escaping a liquid (through evaporation) equals the number of molecules entering it (through condensation). The system is then said to be in equilibrium.
Mathematically it is defined as:
