Plain English Meaning
A piston engine that breathes whatever air the atmosphere gives it, with no pump to squeeze more air in. The higher you fly, the thinner the air, and the less power the engine can produce.
Full Definition
A piston aircraft engine that draws its intake air at ambient atmospheric pressure, with no mechanical compressor (supercharger or turbocharger) to boost manifold pressure. Because intake air density falls as altitude increases, the engine's available power decreases steadily with altitude. Also called a normally aspirated engine.