The term is defined by what is absent. There is no turbine and no compressor between the outside air and the cylinder, so the only thing pushing air in is the atmosphere itself, filling the space the descending piston has just made. Whatever the engine can draw under its own suction is all it gets on that stroke.
The contrast is forced induction, where the US Department of Energy notes that a turbocharger is a compressor that "force[s] additional air into the engine's cylinders". Remove that and you are back to a naturally aspirated engine, which usually needs more displacement to reach the same output.
What it buys is directness. Nothing has to spool up, so the response to the throttle arrives with the throttle, and the power tends to build in a straight, predictable line as the revs rise rather than arriving in a step. That character is why a lot of the engines enthusiasts hold up as the best are naturally aspirated, and why turbocharging is still argued about rather than simply accepted as the upgrade it is on paper. It's also why the horsepower figure alone never settles the argument.
Written by Craig Fearn, Petrol & Ink. Last updated .