- Humidity specifically refers to the concentration of water vapor present in the air
It can be expressed as absolute humidity (mass of water vapor per unit volume or mass of dry air) or relative humidity (ratio of current water vapor pressure to saturation vapor pressure at a given temperature)
- Moisture is a more general term referring to the presence of a liquid, especially water, often diffused or condensed
- Viscosity is a measure of a fluid's resistance to flow
- Density is mass per unit volume
The term specifically connected with the amount of water vapor in air is humidity