Plain English Meaning
An engine that breathes in air on its own, using only the suction of its pistons. Nothing pushes extra air into it, so the higher you fly, the less power it produces.
Full Definition
A reciprocating engine that draws its intake air using only the pressure difference created by the descending pistons, without any mechanical device to compress the air before it enters the cylinders. Because intake air pressure can never exceed the surrounding atmospheric pressure, engine power output decreases as altitude increases and the air becomes less dense.
Context Anchor
Seen in aircraft engine descriptions, performance charts, altitude performance discussions, and maintenance references for aircraft.