**Impedance** is the complex-valued generalization of resistance. It may refer to:

**Acoustic impedance**: a constant related to the propagation of sound waves in an acoustic medium.**Electrical impedance**: the ratio of the voltage phasor to the electric current phasor, a measure of the opposition to time-varying electric current in an electric circuit.- High impedance, when only a small amount of current is allowed through.
- The characteristic impedance of a transmission line.
- Impedance (accelerator physics), a characterization of the self-interaction of a charged particle beam.
- Nominal impedance, approximately designed impedance.
- Impedance matching, the adjustment of input impedance, and output impedance.

**Mechanical impedance**: a measure of opposition to the motion of a structure subjected to a force.**Wave impedance**: a constant related to electromagnetic wave propagation in a medium.