${\displaystyle \underset{\_}{Z}=\frac{{\underset{\_}{U}}_{\text{A}\text{B}}}{\underset{\_}{I}}}$

where the sinusoidal voltage ${\displaystyle u_{\text{A}\text{B}}=v_{\text{A}}-v_{\text{B}}}$ represented by the phasor ${\displaystyle {\underset{\_}{U}}_{\mathrm{AB}}}$ is the difference of the electric potentials ${\displaystyle v_{\text{B}}}$ at A and ${\displaystyle v_{\text{B}}}$ at B, and where the sinusoidal electric current represented by the phasor ${\displaystyle \underset{\_}{I}}$ 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 $\underset{\_}{Z}=R+\mathrm{j}X=Z{\mathrm{e}}^{\mathrm{j}\phi }$, 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.