Impedance is a measure of the opposition that a capacitor presents to the flow of alternating current (AC). Unlike a resistor, which provides a constant opposition to current regardless of frequency, a capacitor’s impedance varies with the frequency of the AC signal.Impedance is a complex quantity, meaning it has both magnitude and phase. At low frequencies, the impedance of a capacitor is relatively high, limiting the flow of current. As the frequency of the AC signal increases, the impedance of the capacitor decreases. Impedance is represented by the symbol “Z” and is measured in ohms.