The dictionary definition for a solenoid is: "a coil of wire usually in cylindrical form that when carrying a current acts like a magnet so that a movable core is drawn into the coil when a current flows and that is used especially as a switch or control for a mechanical device (such as a valve)" (Merriman Webster).
A solenoid is an electromagnetic switching device, used to switch on and off heavy duty circuits such as starting motors or glow plug circuits.
They are similar to relays in construction, operation and use. The main difference is that in relays, we use an electromagnet to close a set of contact points, but in a solenoid we use an electromagnet to move a soft ironarmature into a circuit so that it completes an electrical pathway between two terminals.
Below, we can see a solenoid controlling a glow plug circuit:
The dictionary definition for a solenoid is:
