Question

what is meant by
humidity?

Answer 1

A quantity representing the amount of water vapour in the atmosphere or in gas 

Answer 2

Humidity, in general represents the total amount of water vapour present in the air.
There are three types of humidity:
1. Absolute humidity: it is the total mass of water vapour present in a given volume of air.
2. Specific humidity: It is the ratio of mass of water vapour to the total mass of air and water vapour in a given volume in atmosphere.
3. Relative humidity: It is defined as the ratio of partial pressure of water vapour to the saturated vapour pressure of water at that given temperature.

Among the three, relative humidity is most important. it depends on the temperature of air.