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Capacitive reactance (symbol Xc) is a measure of a capacitor's opposition to AC (alternating current). Like resistance it is measured in ohms, Ω, but reactance is more complex than resistance because its value depends on the frequency (f) of the electrical signal passing through the capacitor as well as on the capacitance, C.

The reactance Xc is large at low frequencies and small at high frequencies. For steady DC which is zero frequency, Xc is infinite (total opposition), hence the rule that capacitors pass AC but block DC.

For example a 1µF capacitor has a reactance of 3.2kΩ for a 50Hz signal, but when the frequency is higher at 10kHz its reactance is only 16Ω.

Note: the symbol Xc is used to distinguish capacitive reactance from inductive reactance XL which is a property of inductors. The distinction is important because XL increases with frequency (the opposite of Xc) and if both XL and Xc are present in a circuit the combined reactance (X) is the difference between them.