Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. 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.
 n. An analogous measure of resistance to an alternating effect, as the resistance to vibration of the medium in sound transmission.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A measure of the opposition to the flow of an alternating current in a circuit; the aggregation of its resistance, inductive and capacitive reactance. Represented by the symbol Z.
 n. a measure of opposition to motion of something subjected to a force.
 n. the sound pressure divided by the particle velocity and the surface area through which an acoustic wave propagates.
 n. a measure of the opposition caused by differences between two paradigms, especially between objectoriented development and relational databases
from the GNU version of the Collaborative International Dictionary of English
 n. 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 R2 + X2, where R = ohmic resistance, X = reactance. For an inductive circuit, X = 2πfL, where f = frequency and L = selfinductance; for a circuit with capacity X = 1 ÷ 2πfC, where C = capacity.
from The Century Dictionary and Cyclopedia
 n. Hindrance; specifically, in electricity, an apparent increase of resistance due to induction in a circuit.
 n. Impedance is the apparent resistance of an alternatingcurrent circuit, or the ratio of the electromotive force consumed by an alternating current, divided by the current. On account of the electromotive force consumed by selfinduction, the apparent resistance with alternating currents is greater than that with direct currents, though the powerconsumption is the same. The impedance consists of two components—resistance, which consumes power, and reactance, which is the wattless resistance due to selfinduction. See resistance, 3, and reactance.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. a material's opposition to the flow of electric current; measured in ohms
Etymologies
from Wiktionary, Creative Commons Attribution/ShareAlike License
Examples

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

One of the problems the middle ear has to overcome is the mismatch in impedance between air and the cochlea, but impedance matching by the middle ear results in only a 30 dB increase in sound energy transmitted to the inner ear.

Guts: One of the problems the middle ear has to overcome is the mismatch in impedance between air and the cochlea, but impedance matching by the middle ear results in only a 30 dB increase in sound energy transmitted to the inner ear.

Yes, the difference in impedance would be fairly large.

The phrase impedance matching seems to have moved towards mainstream language in recent times:

When effective resistances and reactances are connected together, either in series or parallel or seriesparallel, then the equivalent resistance of the overall circuit is called impedance Z.

The issue with the Thiels is they have low sensitivity (84dB, I believe for the 3.6) and impedance, which is rated at 4 ohm but may actually be as low as 3.

The music reference hit home as well, as I often experienced this kind of impedance mismatch with the drummer from my old band  he's an inexhaustible fountain of music minutiae.

Furthermore expertise in a number of relative applied techniques, such as impedance spectroscopy will also be vital.

Note: For the purposes of this explanation the term "impedance" is synonymous with "electrical resistance".
fbharjo commented on the word impedance
another dance other than hoppestere
July 14, 2010