A solenoid is a device that converts electrical energy into mechanical energy using an electromagnet formed by a coil of wire. It typically consists of a helical coil of wire, a housing (often iron or steel), and a movable plunger (or armature) made of magnetic material. When an electric current passes through the coil, it creates a magnetic field that moves the plunger inside the coil. This movement produces mechanical work such as pushing or pulling forces. More simply, a solenoid is an electromechanical device where an electric current flowing through the coil generates a magnetic field that causes mechanical motion of the core or plunger. This motion can be used to operate switches, valves, or other mechanical parts. The coil forms a controlled magnetic field in a volume of space when energized. The magnetic field produced can be strong and uniform, and the strength and direction of the motion can be controlled by varying the electric current. When the current is switched off, springs or other mechanisms usually return the plunger to its original position. Solenoids are commonly found in devices like car starters, doorbells, valves for controlling fluid flow, and many other applications requiring conversion of electrical energy into mechanical movement. In summary:

• A solenoid is a coil of wire that creates a magnetic field when electrically energized.
• It has a movable core or plunger that moves due to the magnetic field.
• The movement is used to perform mechanical work.
• Solenoids convert electrical energy into linear or rotary mechanical motion.

The term solenoid was coined by André-Marie Ampère in 1823 and originally meant "tubular," relating to the coil shape.