| for a passive linear two-terminal element or two-terminal circuit with terminals A and B under sinusoidal conditions, quotient of the phasor representing the voltage (131-11-56) between the terminals by the phasor representing the electric current in the element or circuit
where the sinusoidal voltage represented by the phasor is the difference of the electric potentials at A and at B, and where the sinusoidal electric current represented by the phasor is taken positive if its direction is from A to B and negative if its direction is from B to A
Note 1 to entry: The impedance of an element or circuit is the inverse of its admittance. It is equal to , where R is resistance to alternating current, X is reactance, Z is apparent impedance, and φ is displacement angle.
Note 2 to entry: With a suitable qualifier, the word impedance is used to form composite terms designating quantities of the same kind as impedance, e.g.: transfer impedance, characteristic impedance.
Note 3 to entry: The coherent SI unit of impedance is ohm, Ω.