1. An electrically energized coil of insulated wire that produces a magnetic field within the coil; in particular, a coil that surrounds a movable iron core that is pulled to a central position with respect to the coil when it is energized by sending electric current through the coil.
2. A device made of a long wire which has been wound around many times into a tightly packed coil.

It has the shape of a long cylinder and if a current is sent through a solenoid made of insulated wire and having a length much greater than its diameter, a uniform magnetic field will be created inside the solenoid.

This field can be intensified by inserting a ferromagnetic core into the solenoid.