- Opposition to alternating current (AC) by capacitance or inductance or both. It is expressed in ohms, and symbolized by X.
- The imaginary number component of impedance. It is the part of the total impedance which is not due to pure resistance. When reactance is due to a pure capacitance it is called capacitive reactance, and when due to a pure inductance it is an inductive reactance. Expressed in ohms. Its symbol is X.