Reactance is a circuit element's opposition to an alternating current, caused by the build up of electric or magnetic fields in the element due to the current. Both fields act to produce counter emf that is proportional to either the rate of change (time derivative), or accumulation (time integral) of the current. In vector analysis, Reactance is the imaginary part of electrical impedance, used to compute amplitude and phase changes of sinusoidal alternating current going through the circuit element. It is denoted by the symbol . The SI unit of reactance is the ohm.
Both reactance and resistance are required to calculate the impedance , although in some circuits one of these may dominate: an approximate knowledge of the minor component is useful to determine if it may be neglected.
Both the magnitude