A coil of wire that generates a magnetic field when current is passed through it. The strength of the magnetic field is measured in henrys (H). When the current is removed, as the magnetic field disintegrates, it "induces" a brief current in the opposite direction of the original. Thus "electromagnetic induction" is caused by the opening and closing of a DC circuit or the continuous changing of directions in an AC circuit.
An induction coil impedes the flow of high-frequencies in an AC circuit, which is why inductors are used as surge protectors, choking off any high-frequency shifts. The tiny donuts placed on the end of signal cables and the load coils placed into telephone networks are examples. See electromagnetic induction