(ĭm-pēd'ns) pronunciation
  1. (Symbol Z) A measure of the total opposition to current flow in an alternating current circuit, made up of two components, ohmic resistance and reactance, and usually represented in complex notation as Z = R + iX, where R is the ohmic resistance and X is the reactance.
  2. An analogous measure of resistance to an alternating effect, as the resistance to vibration of the medium in sound transmission.

impedance (im-PEED-ns)

A measure of the apparent resistance posed by an electrical circuit to an alternating current (AC).

• The term impedance is most often encountered in dealing with antennas and speakers in television, stereo, and radio systems.

(1) The opposition to the flow of alternating current in a circuit. Represented by the letter "Z" and measured in ohms, impedance is the combination of resistance, inductance and capacitance of the circuit. See ohm.

(2) The opposition that a speaker produces to the alternating current coming from an amplifier. The lower the impedance, the more power required. Most speakers have an impedance of four to eight ohms. See ohm.

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impedance, in electricity, measure in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals. Impedance is expressed as the ratio of the voltage impressed across a pair of terminals to the current flow between those terminals. When a circuit is supplied with steady direct current, the impedance equals the total resistance of the circuit. The resistance depends upon the number of electrons that are free to become part of the current and upon the difficulty that the electrons have in moving through the circuit. When a circuit is supplied with alternating current, the impedance is affected by the inductance and capacitance in the circuit. When supplied with alternating current, elements of the circuit that contain inductance or capacitance build up voltages that act in opposition to the flow of current. This opposition is called reactance, and it must be combined with the resistance to find the impedance. The reactance produced by inductance is proportional to the frequency of the alternating current. The reactance produced by capacitance is inversely proportional to the frequency of the alternating current. In order for a source of electricity that has an internal impedance to transfer maximum power to a device that also has an impedance, the two impedances must be matched. For example, in the simple case of pure resistances, the resistance of the source must also equal the resistance of the device. Impedance matching is important in any electrical or electronic system in which power transfer must be maximized.


(Z) Measured in ohms it is the total opposition to the flow of current offered by a circuit. Impedance consists of the vector sum of resistance and reactance.

symbol: Z; the quality determining the amplitude of the current flowing in a circuit for a given applied alternating electric potential. It depends upon the electric resistance, self-inductance, and capacitance of the circuit.

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Obstruction or opposition to passage or flow, as of an electric current or other form of energy.

  • acoustic i. — an expression of the opposition to passage of sound waves, being the product of the density of a substance and the velocity of sound in it.
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Common misspelling(s) of impedance

  • impedence

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Mentioned in

Z0 (physics)
auditory impedance (physiology)
image admittance (electronics)
normalized admittance (electromagnetism)