Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

impedance
     n : a material's opposition to the flow of electric current;
         measured in ohms [syn: {electric resistance}, {electrical
         resistance}, {resistance}, {resistivity}, {ohmic
         resistance}]