·add. ·noun The influence of a coil of wire upon an alternating current passing through it, tending to choke or diminish the current, or the similar influence of a condenser; inductive resistance. Reactance is measured in ohms. The reactance of a circuit is equal to the component of the impressed electro-motive force at right angles to the current divided by the current, that is, the component of the impedance due to the self-inductance or capacity of the circuit.