Plain English Meaning
Restarting an engine that has stopped while the aircraft is still flying.
Full Definition
The procedure of starting an aircraft engine while in flight, typically following an in-flight shutdown or flameout. Airstarts may be accomplished using the engine's normal starter, by windmilling (allowing airflow through the engine to spin it), or a combination of both, depending on the aircraft type and the published airstart envelope (airspeed and altitude limits).
Context Anchor
You may see this term in emergency procedures, engine-failure discussions, and aircraft checklists that describe how to restart an engine in flight.