The partial pressure of a gas in equilibrium with a condensed form (solid or liquid) of the same substance.
A colligative property of solutions.
The vapour pressure of a solution is always lower than the vapour pressure of the pure solvent; the ratio of solution to pure solvent vapour pressures is approximately equal to the mole fraction of solvent in the solution.
Vapour pressure is considered as the part of total atmospheric pressure due to the water vapour content. It is independent of other gases or vapours.