Engine is a machine that can convert other forms of energy into mechanical energy, including internal combustion engines (reciprocating piston engines), external combustion engines (Stirling engines, steam engines, etc.), jet engines, motors, etc. For example, internal combustion engines usually convert chemical energy into mechanical energy.

Engine is not only applicable to power generation device, but also refers to the whole machine including power device (such as gasoline engine and aeroengine). The engine was first born in Britain, so the concept of engine also originates from English, and its original meaning refers to the kind of "mechanical device that generates power".