Reactance in conductors and capacitors is measured in ohms (Ω). It describes opposition to alternating current caused by inductance or capacitance and forms the imaginary part of a circuit’s impedance.
Conductors possess inductance because changing current creates a changing magnetic field around them. This produces inductive reactance, which increases with frequency. A capacitor stores energy in an electric field, creating capacitive reactance, which decreases as frequency rises. Both effects are expressed in ohms because reactance relates voltage and current in the same dimensional way as resistance.
Reactance is not the same as resistance. Resistance converts electrical energy into heat, while ideal reactance temporarily stores energy and returns it to the circuit. It also shifts the phase between voltage and current. Inductive reactance is conventionally positive, whereas capacitive reactance is negative when treated as the imaginary part of impedance.
A common mix-up is confusing ohms with the units of the underlying component: inductance is measured in henries and capacitance in farads. The reactance calculated from those values, together with frequency, is measured in ohms. The term “reactance” was proposed by Édouard Hospitalier in 1893.