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Actually, the term solenoid comes from the science of physics and refers to a coil of wire that can produce a magnetic field. In engineering it generally refers to a coil of wire used as an electromagnet to control the linear movement of something. A relay is a specific kind of solenoid that uses a small electric current in the electromagnet coil to move the contacts for a switch that can then control a much larger amount of current. The term solenoid is also used in hydraulics and pneumatics to refer to electromagnetically controlled hydraulic and pneumatic control valves.