The Collaborative International Dictionary of English v.0.48:

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 rootR^{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.]

WordNet (r) 3.0 (2006):

impedance
    n 1: a material's opposition to the flow of electric current;
         measured in ohms [syn: electric resistance, electrical
         resistance, impedance, resistance, resistivity,
         ohmic resistance]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "impedance":
   capacitive reactance, clog, cumbrance, electric resistance,
   hindrance, impediment, inductive reactance, magnetic reluctance,
   ohm, ohmage, reactance, reluctance, reluctivity, resistance,
   skin effect, specific reluctance, surface resistance,
   volume resistance



The Free On-line Dictionary of Computing (30 December 2018):

impedance

    Opposition to flow of alternating
   current.  Impedance consists of resistance plus reactance
   (capacitive or inductive).  Measured in Ohms.

   (2003-12-02)