Plain English Meaning
An airplane with just one engine. If the engine quits, there is no second engine to keep you flying.
Full Definition
An airplane powered by one engine, which provides all of the thrust needed for flight. If that engine fails, the airplane has no other powerplant to maintain thrust and must glide to a landing.
Context Anchor
Seen in aircraft descriptions, training requirements, and discussions of stalls, takeoffs, landings, and engine-out procedures.
Why Pilots Care
Single-engine airplanes lack engine redundancy, so pilots must master specific emergency procedures for engine failure.