Plain English Meaning
It is the matched set of an airplane body and the engine fitted to it. The two are tested and approved together, so the numbers in the handbook only apply when that exact body and that exact engine are paired.
Full Definition
The specific pairing of a particular airframe with a particular engine model, treated as an approved unit by the manufacturer and the FAA. Performance data, operating limitations, and recommended procedures are published for that exact pairing, because changing either the airframe or the engine changes how the aircraft performs and what it is certified to do.
Context Anchor
Seen in powerplant, propeller, performance, and aircraft approval discussions where the handbook is explaining what equipment is correct for a particular aircraft.