This term is often used by car enthusiasts to describe a car engine that is not enhanced by any form of forced induction, meaning no turbocharger, no supercharger, and no nitrous oxide. Also, the term "atmospheric" is often used, because n/a engines are running at atmospheric pressure.
Generally, naturally aspirated cars must work harder than a car with forced induction, setting them at a disadvantage. Many new technologies have arisen in recent years to combat this problem, including the use of variable valve timing technology. Honda's VTECtm, Ford's Duratectm, and Chevy's Vortectm are examples. Another recent trend is the use of dual overhead cam engines, which generally have twice as many valves as a standard single overhead cam or old-fashioned pushrod (or overhead valve) engine. A normal SOHC engine generally has one intake and one exhaust valve for every cylinder in the engine, whereas a DOHC engine has twice as many, allowing it to "breathe" more rapidly and rev to higher RPMs. Some companies like Mercedes, BMW, Ferrari, and Porsche/Audi/VW have taken to adding an additional exhaust valve, in either a three or five valve per cylinder configuration.