Vapor Pressure
Definition
The pressure exerted by a liquid's vapor when evaporation and condensation balance out. High vapor pressure means the liquid evaporates easily.
Technical definition
Vapor pressure is the equilibrium pressure of a vapor in contact with its liquid at a given temperature, when evaporation and condensation rates are equal. It rises with temperature and falls with stronger intermolecular forces.
Example
At 20 C, water's vapor pressure is about 2.3 kPa while ethanol's is about 5.9 kPa. Ethanol evaporates faster, which is why it feels colder on skin.
Common confusion
Vapor pressure does not depend on how much liquid is present. A thimble of water and a swimming pool at the same temperature have the same vapor pressure.