Fraction expressing mass of component per total mass of substance present. May be written with no units (said to be 'dimensionless'), for example 'water vapour in nitrogen at a mass fraction of 0.002 (or 2 parts in 10^3)'. Alternatively in mass units such as 'kilograms of water per kilogram of (humid) gas'.

Mass of water vapour per unit mass of dry air. May be written with no units (said to be 'dimensionless'), but often expressed in grams of water per kilogram of dry gas.

Fraction expressing volume of component per total substance present. May be written with no units (said to be 'dimensionless'), for example 'water vapour in nitrogen at a volume fraction of 5 parts per million (or 5 parts in 10⁶)'.

Amount of substance which contains as many elementary entities as there are atoms in 12 grams of carbon 12. Expressed in moles (symbol: mol).

NOTE: When the mole is used, the elementary entities must be specified as atoms, molecules etc.

Amount (number of moles) of a component as a fraction of the total amount of substance present. Also known as 'mole fraction'. May be written with no units (said to be 'dimensionless', or of 'dimension 1') for example 'water vapour in nitrogen at a mole fraction of 0.005 (or 5 parts in 10³)'. Alternatively in 'moles of water per mole of gas'.

Informal way of expressing trace water content (amount fraction or ratio; mass fraction or ratio; or volume fraction or ratio). '1 ppm' means '1 part per million' or '1 part in 10^6'. 

The amount of mass of water vapour per unit volume.

For example, room air might typically contain about 10 grams of water vapour per cubic metre.