See impedance in Webster's New World College Dictionary
noun
the total opposition offered by an electric circuit to the flow of an alternating current of a single frequency: it is a combination of resistance and reactance and is measured in ohms: the reciprocal of admittance: symbol, Z
the ratio of the force per unit area to the volume displacement of a given surface across which sound is being transmitted
See impedance in American Heritage Dictionary 4
(ĭm-pēdˈns)
noun
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.
An analogous measure of resistance to an alternating effect, as the resistance to vibration of the medium in sound transmission.