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A turbofan engine, sometimes referred to as a fanjet or bypass engine, is a jet engine variant which produces thrust using a combination of jet core efflux and bypass air which has been accelerated by a ducted fan driven by the jet core. The ratio of the mass of air bypassing the engine core versus the mass of the air going through the core is referred to as the bypass ratio. A turbofan engine which derives most of its thrust from the jet engine core efflux are referred to as low bypass engines whereas an engine that derives most of its thrust from the fan are referred to as high bypass engines. In general, low bypass engines are most commonly found in military applications whereas high bypass turbofan engines are the prevalent design in today's commercial aviation jet engines.