Impedance is the opposition to current flow in an AC circuit. The ratio of voltage and current indicates it, and the unit is "Ω" as with electrical resistance. In a DC circuit, coils and capacitors do not interfere with the current.

In AC circuits, however, not only resistors, but also coils, capacitors, and the circuit itself act like resistors to block current.
As for a coil, if the voltage is constant, the current is also constant. However, when the voltage is not constant, electromagnetic induction tries to oppose the change, resulting in a phenomenon that interferes with the current. Because of it, the coil will be an impedance.

Thus, things that interfere with current in an AC circuit are all regarded as impedance.
Impedance has the property of resistance. It does not change the values by the frequency of electrical resistance. On the other hand, reactance is the property that changes values with frequency. The use of self and mutual impedances in a circuit that could cause equipment malfunction requires particular attention.