A turbocharger, or turbo (colloquialism), from the Greek "τύρβη" (mixing/spinning) is a centrifugal compressor powered by a high speed turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine (forced induction), thereby resulting in greater performance (for either, or both, power and efficiency). They are popularly used with internal combustion engines (e.g., four-stroke engines like Otto cycles and Diesel cycles). Turbochargers have also been found useful compounding external combustion engines such as automotive fuel cells.

The term turbocharger is a modern one, derived by shortening the turbosupercharger, which was widely used during the World War II era and earlier. This term refers to the fact that turbochargers are a specific type of supercharger, one that is driven by a turbine. The most common form of supercharger at the time, which was often referred to as a "geared supercharger", was mechanically driven by the engine, whereas turbochargers are always driven by a turbine that gets its power from the engine's exhaust stream. Twinchargers combine a supercharger and turbocharger.

Turbochargers are also employed in certain two-stroke cycle diesel engines, which would normally require a Roots blower for aspiration. In this specific application, mainly Electro-Motive Diesel (EMD) 567, 645, and 710 Series engines, the turbocharger is initially driven by the engine's crankshaft through a gear train and an overriding clutch, thereby providing aspiration for combustion. After the engine achieves combustion, and after the exhaust gases reach sufficient temperature, the overriding clutch disengages the turbo-compressor from the gear train and the turbo-compressor is thereafter driven exclusively by the turbine, which, in turn, is driven by the exhaust gases. In the EMD application, the turbocharger is used for normal aspiration during starting and low power output settings and is used for true turbocharging during medium and high power output settings. This is particularly beneficial at high altitudes, as are often encountered on western U.S. railroads. One EMD engine model was fitted with a "locked" turbocharger; it was used in normal aspiration mode during starting and all power output settings.