According to Webster's New Collegiate Dictionary, a solenoid is "a tubular coil for the production of a magnetic field." This magnetic field is what moves a spring-loaded steel or iron plunger within the solenoid when an electrical current is present.
In automotive use, a solenoid is usually a heavy-duty electrical relay. The plunger opens or closes a set of electrical contacts when a small control current is present in the coil.
The most common use for a solenoid in a car is to control the starter motor. The starter must produce enough power to rapidly spin the engine until it starts, and this requires a large electrical current. Because a car's ignition switch is not large enough to carry enough current to operate the starter, the ignition switch controls the solenoid, which can produce the necessary current.
Most cars have a variety of valves to control fluid or vacuum in the transmission, climate control, and emissions control systems. When such a valve uses an electrical impulse for activation, it is called a solenoid.
