Any coiled device, with or without a magnetic core, used to introduce inductive reactance into a circuit. An inductor is an energy storing device. Inductors oppose any change in current. Inductors are the mirror-image counterparts of capacitators (which resist voltage changes). Inductive reactance is defined as XL = 6.28 fL or XL = 2(pi)fL. Inductors block AC and pass DC.