Vapor pressure is the amount of pressure which is exerted by the vapor phase of a liquid that has undergone some evaporation.

When the rate of evaporation of the liquid into the vapor phase and the rate of condensation of the vapor back into the liquid phase are equal, the amount of pressure is called the equilibrium vapor pressure. A liquid which has no measurable vapor pressure (and barely evaporates) is "nonvolatile." Vapor pressure (and volatility) increases with increasing temperature.

The information in this writeup was taken from the science dictionary at; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.