Symbol X. A property of a circuit containing inductance or capacitance that together with any resistance makes up its impedance. The impedance Z is given by Z2=R2+X2, where R is the resistance. For a pure capacitance C, the reactance is given by XC=1/2πfC, where f is the frequency of the alternating current; for a pure inductance L, XL=2πfL. If the resistance, inductance, and capacitance are in series the impedance Z=√[R2+(XL − XC)2]. Reactance is measured in ohms.