A solenoid is an electical devise that engages one gear to another and then rotates the second gear by rotating the first. For example, a starter solenoid thrusts a small, exterenal toothed gear into the internal teeth of a flywheel and then turns the flywheel in order to crank the engine until it starts and run by itself.
it's a valve that's opened and/or closed by use of electricity to an electromagnetic solenoid coil. Used to control air/gases like the EVAP purge valve or fluids like in an automatic transmission for convertor clutch lockup, or shifting.
An electric switch, like the one on your cars starter motor.
