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the ratio of voltage to electric current, as in

Electrical impedance, a measure of opposition to time-varying electric current in an electric circuit.

Characteristic impedance, a measure of opposition to electric current propagation in a transmission line.

Electromagnetic impedance, a measure of opposition to electromagnetic wave propagation in a medium.

Mechanical impedance, a measure of opposition to motion of a structure subjected to a force.

Acoustic impedance, a measure of opposition to the propagation of sound waves in an acoustic medium.