Capacitors can conduct direct current for only an instant but function well as conductors in alternating-current circuits. This property makes them useful when direct current must be prevented from entering some part of an electric circuit. It is the resistance to the sinusoidal AC wave form. In the figure to the left XL is inductive reactance, XC is capacitive reactance, R is resistance, and Z is impedance. The two kinds of reactance, inductive and capacitive, nullify each other.
Inductors resist changes to current flow and capacitors resist changes to voltage.
Reactance is different than resistance, but like resistance it is measured in ohms. Resistance resists the flow of electrons through a material, not changes in the voltage or current. Reactance depends upon the frequency and the inductance or capacitance. Impedance of a circuit is important. It is the resistance to both the flow of electrons and to changes in voltage or current.