Air mass

"A widespread body of air, the approximately homogeneous properties of which (1) have been established while that air was situated over a particular region of the Earth’s surface, and (2) undergo specific modifications while in transit away from the source region" (AMS, 2000).

Broadly speaking, a parcel of air that moves together and is changed by the regions it passes through in the same way

Definition based on (revised from) IPCC AR4.