Vapor pressure is the pressure exerted by a vapor in a vapor-liquid equilibrium in a closed vessel. The relationship between a pure substance's vapor pressure and its temperature is shown on the liquid-vapor line on a phase diagram; the vapor pressure always increases as the temperature increases. 
When the pressure on a system is equal to the vapor pressure of a substance, that substance is considered boiling. Thus, the normal boiling temperature is defined as the the temperature at which a substances vapor pressure equals 1 atmosphere.
- ↑ Atkins, Peter and Julio de Paula. Physical Chemistry Volume 1, 8e. Oxford University Press: 2006.