Dalton's Law (P_m)
John Dalton was first to surmise that the total pressure (Pm) exerted by a mixture of gases or vapors is the sum of the pressures of each gas if it were to occupy the same volume by itself. The pressure which each gas exerts is called its partial pressure. If Px, Py and Pz represent the respective partial pressures of gases X, Y and Z in a mixture, Dalton’s law states: 

P_m = P_x + P_y + P_z + …

The concept of Dalton’s law is often overlooked in considering problems in humidity measurement, because one forgets that the “water” in a gas is actually a gas itself and must be treated in accordance with the gas laws. Air must be considered a mixture of gases – oxygen, nitrogen ,and water vapor (neglecting the minor constituents). All discussions and definitions of humidity can be reduced to discussions and definitions of water vapor pressure.

Saturation Vapor Pressure Dew Point (P_ws)
Saturation vapor pressure Pws is the maximum water vapor pressure that can exist at a given temperature.

Dew Point (DP)
Dew point (DP) is that unique temperature to which the air (or any gas) must be cooled in order that it shall be saturated with respect to water.

Frost Point (FP)
Frost point (FP) is that unique temperature to which the air (or any gas) must be cooled in order that it shall be saturated with respect to ice.

*Note: the dew point or frost point DEFINES the partial pressure of water vapor in the gas.