A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid shows  a long, thin loop of wire, often wrapped over a metallic core, which makes a magnetic field when an electric current is passed by it. Solenoids are important because they can make controlled magnetic fields and can be used as electromagnets. The term solenoid refers specifically to a coil designed to create a uniform magnetic field in a volume of space (where some experiment might be carried out).

In engineering, the term solenoid may also refer to a variety of transducer devices that change energy into linear motion. The term is also often used to refer to a solenoid valve, which is an integrated device having an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for instance, an automobile starter solenoid, or a linear solenoid, which is an electromechanical solenoid.