Plain English Meaning
A pump driven by the engine itself squeezes extra air into the cylinders so the engine can keep making strong power even as the airplane climbs into thinner air.
Full Definition
A method of increasing engine power by using an engine-driven air compressor to force more air into the cylinders than they would draw in naturally, allowing more fuel to be burned and maintaining sea-level power at higher altitudes where the air is thinner.
Context Anchor
Seen in performance discussions about engine power, altitude, climb, and range in airplanes equipped with supercharged engines.