It's the process by which gas molecules hitting the surface of the liquid get stuck in the liquid. Now, when the rates of evaporation equal condensation, the state is an equilibrium. And when the state is in the equilibrium, we can measure the pressure exerted by the gas molecules on the wall of the container. And this pressure is called vapor pressure. So vapor pressure, by definition, is the pressure exerted by gas molecules that are in equilibrium with their liquid or solid state. And at this point, the rate of molecules leaving the solid or liquid state is equal to the rate at which they enter that state.