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The Collaborative International Dictionary of English v.0.48 (gcide)

Impedance \Im*ped"ance\, n. [Impede -ance.] (Elec.)
The apparent resistance in an electric circuit to the flow of
an alternating current, analogous to the actual electrical
resistance to a direct current, being the ratio of
electromotive force to the current. It is equal to root{R^{2}
X^{2}}, where R = ohmic resistance, X = reactance. For an
inductive circuit, X = 2[pi]fL, where f = frequency and L =
self-inductance; for a circuit with capacity X = 1 [div]
2[pi]fC, where C = capacity.
[Webster 1913 Suppl.]