Adding solute molecules to a solution lowers this quantity but raises the boiling point, which is defined as the temperature at which this quantity equals the external pressure.
For a solid, it can be determined using the Knudsen method or thermogravimetry.  One can use the Kelvin equation to calculate it for a single drop of liquid, and the Goff-Gratch equation is the meteorological standard for calculating it for water. If the value of this quantity is known at one temperature, the Clausius-Clapeyron equation can be used to find its value at any other temperature, and this value can be calculated as the product of its value for a pure solvent and the solvent's mole fraction in Raoult's Law.  Adding solute molecules to a solution lowers this quantity but raises the boiling point, which is defined as the temperature at which this quantity equals the external pressure. FTP name this quantity defined as the pressure of a gas in equilibrium with a confined liquid or solid.
For a solid, it can be determined using the Knudsen method or thermogravimetry.  
vapor pressure