Reactance is a measure of the opposition presented to alternating current by inductance and capacitance in electrical circuits. It is one of two elements of impedance, the other being resistance. Reactance is used to compute amplitude and phase changes of sinusoidal alternating current going through a circuit element. Like resistance, reactance is measured in ohms, with positive values indicating inductive reactance and negative indicating capacitive reactance.

There are several important differences between reactance and resistance. First, reactance changes the phase so that the current through the element is shifted by a quarter of a cycle relative to the phase of the voltage applied across the element. Second, power is not dissipated in a purely reactive element but is stored instead. Third, reactances can be negative so that they can cancel each other out. Finally, the main circuit elements that have reactance (capacitors and inductors) have a frequency-dependent reactance, unlike resistors which have the same resistance for all frequencies.

Inductive reactance is associated with the magnetic field that surrounds a wire or a coil carrying a current. An alternating current in such a conductor, or inductor, sets up an alternating magnetic field that in turn affects the current in, and the voltage (potential difference) across, that part of the circuit. Capacitive reactance is defined as the reactance produced due to the capacitive elements (capacitor) .

In summary, reactance is a measure of the opposition presented to alternating current by inductance and capacitance in electrical circuits. It is used to compute amplitude and phase changes of sinusoidal alternating current going through a circuit element. Reactance differs from resistance in several ways, including the fact that it changes the phase of the current and stores power instead of dissipating it. Inductive reactance is associated with the magnetic field that surrounds a wire or a coil carrying a current, while capacitive reactance is produced due to the capacitive elements.