1. A turbine that generates power from the combustion of a fuel.
2. A heat engine that converts the energy of fuel into work by using compressed, hot gas as the working medium and which usually delivers its mechanical output power either as torque through a rotating shaft (industrial gas turbines) or as jet power in the form of velocity through an exhaust nozzle (aircraft jet engines).
3. An internal combustion engine in which liquid or gaseous fuel is used to generate mechanical energy through a rotating shaft, which then drives an electric generator or another piece of equipment.
4. One of a class of heat engines that use fuel energy to produce mechanical output power, either as torque through a rotating shaft (industrial gas turbines) or as jet power in the form of velocity through an exhaust nozzle (aircraft jet engines).

The fuel energy is added to the working substance, that is gaseous in form and most often air, either by direct internal combustion or indirectly through a heat exchanger.