noun. a widespread body of air, the properties of which can be identified as: (a) having been established while that air was situated over a particular region of the earth's surface (airmass source region) and (b) undergoing specific modifications while in transit away from the source region. An air mass is often defined as a widespread body of air that is approximately homogeneous in its horizontal extent, particularly with reference to temperature and moisture distribution; in addition, the vertical temperature and moisture variations are approximately the same over its horizontal extent.

"The top of the “anvil” that you see at the top of a thunderstorm is actually the level at which buoyant parcels reach equilibrium with the surrounding airmass, which is generally isothermal (in the vertical) at that height (i.e., the tropopause)."

Air mass classification  airmass source region