Humidity - Absolute, Specific and Relative Humidity

Humidity is a very general term. Humidity is the amount of water vapour present in air.
Absolute Humidity is the ratio of mass of water vapour to the volume of air & water vapor mixture
Specific Humidity is the ratio of mass of water vapour to the total mass of air.
Relative Humidity is the ratio between the actual quantity of water vapour present in the air at a given temperature and the maximum quantity of water vapour that the atmosphere can hold at that temperature.