Plain English Meaning
An engine that breathes in outside air on its own, with no pump or compressor helping it. The higher you go, the thinner the air, and the less power the engine can make.
Full Definition
Describes a piston aircraft engine that draws its intake air at ambient atmospheric pressure, without the use of a turbocharger or supercharger to compress the incoming air. As altitude increases, air density decreases, so a normally aspirated engine produces progressively less power the higher it flies.
Context Anchor
Seen in climb-performance discussions, especially when explaining why engine power decreases as altitude increases.