Plain English Meaning
A one-stage air pump, driven by the engine, that squeezes thin high-altitude air before it reaches the cylinders so the engine can keep making normal power.
Full Definition
An engine-driven air compressor that uses one impeller to raise the pressure of the air entering the cylinders, allowing the engine to maintain power at altitudes where the surrounding air is too thin to produce rated output.
Context Anchor
Seen in aircraft engine discussions about superchargers, manifold pressure, and how piston engines keep power at higher altitudes.