In acoustics, the product of velocity times density, also called acoustic impedance and symbolized by Z. The reflection coefficient of an interface depends on the contrast in acoustic impedance of the rock on either side of the interface.

In electromagnetics or electrical circuit theory, the ratio of voltage to current when these are represented by phasor quantities in alternating current circuits. (A phasor is a complex number that represents the amplitude and phase of a quantity that varies sinusoidally in time.) Electrical impedance, also symbolized by Z, is a complex number that has the same units (ohms) as resistivity.