Definition: Nitro or glow engines use nitro fuel but it's actually a mixture of fuel and air that goes into the engine. The right air/fuel mixture keeps the engine running at its best. Described as lean or rich, you control the air/fuel mixture by on a nitro RC adjusting the high-end (high speed / engine temperature) and low-end (low speed / idle speed) needles on the carburetor.
See What are lean and rich as applied to Nitro RCs? for a detailed description on when and how to adjust the air/fuel mixture.
Also Known As: fuel/air mixture