A starter (also self starter, self, or starter motor) is an electric motor, pneumatic motor, hydraulic motor, an internal-combustion engine in case of very large engines or other device used for rotating an internal-combustion engine so as to initiate the engine’s operation under its own power.
Engines are feedback systems which once started, rely on the inertia from each cycle to initiate the next cycle. In a four-stroke engine, the third stroke releases energy from the fuel, powering the fourth (exhaust) stroke and also the first two (intake, compression) strokes of the next cycle, as well as powering the engine’s external load. To start the first cycle of engine’s run session, the first two strokes must be powered in some other way. The starter motor is used for this purpose and is not required once the system starts running.