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.