a coil of wire, usually cylindrical, in which a magnetic field is set up by passing a current through it
2.
a coil of wire, partially surrounding an iron core, that is made to move inside the coil by the magnetic field set up by a current: used to convert electrical to mechanical energy, as in the operation of a switch
3.
such a device used as a relay, as in a motor vehicle for connecting the battery directly to the starter motor when activated by the ignition switch
[C19: from French solénoïde, from Greek sōlēn a pipe, tube]