·add. ·noun The apparent resistance in an electric circuit to the flow of an alternating current, analogous to the actual electrical resistance to a direct current, being the ratio of electromotive force to the current. It is equal to R2 + X2, where R = ohmic resistance, X = reactance. For an inductive circuit, X = 2/fL, where f = frequency and L = self-inductance; for a circuit with capacity X = 1 / 2/fC, where C = capacity.