An Air Mass is a large volume of air with certain meteorological or polluted characteristics e.g., a Heat inversion or smogginess--while in one location.

The characteristics can change as the air mass moves away.

Other /More definition:
An Air Mass is a measure of how far light travels through the Earth's atmosphere. One air mass, or AM1, is the thickness of the Earth's atmosphere. Air Mass zero (AM0) describes solar irradiance in space, where it is unaffected by the atmosphere. The power density of AM1 light is about 1,000 W/m_; the power density of AM0 light about 1,360 W/m_.