The Saturn AL-31 is a family of axial flow turbofan engines, developed by the Lyulka design bureau in the Soviet Union, now NPO Saturn in Russia, originally as a 12.5-tonne powerplant for the Sukhoi Su-27 long range air superiority fighter. The AL-31 currently powers the Su-27 family of combat aircraft and some variants of the Chengdu J-10 multirole jet fighter. Assembly of the engine is also performed under license in India by HAL, for the Sukhoi Su-30MKI. Improved variants power the fifth-generation Sukhoi Su-57 and Chengdu J-20.

A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of the preceding generation engine technology of the turbojet, and a reference to the additional fan stage added. It consists of a gas turbine engine which achieves mechanical energy from combustion, and a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.