Plain English Meaning
A huge volume of air with similar conditions throughout that drifts across the ground as one big chunk. The airplane flies through this moving body of air, so when the air mass moves, the airplane is carried along with it.
Full Definition
A large body of air that has relatively uniform properties of temperature, moisture, and density across its horizontal extent, and which moves over the surface of the earth as a single body.
Context Anchor
Used when explaining wind drift, ground track, and why an airplane’s nose may point one way while its path over the ground moves another way.
Derivation
From Latin 'massa' meaning a lump or body of material. The term simply names air as a single body of substance — a useful idea because the airplane is not flying through empty space, it is flying inside a body of moving air.