Electrical resistors have constant, frequency-independent impedances.
Electrical capacitors and inductors, however, have current-to-voltage characteristics that change with respect to the frequency of an applied source.
For mechanical systems, impedance is defined as the ratio of force to velocity.
The inverse of impedance is called admittance. One can use the term immittance to refer to either an impedance or an admittance.
In lossless systems, an immittance is purely imaginary and called a reactance.
Immittances are steady state characterizations that imply zero initial conditions for elements with “memory” (masses and springs, capacitors and inductances).
In acoustics, impedance is given by pressure divided by either particle or volume velocity.
More generally, we can derive (or measure) frequency-domain transfer function representations of vibrating systems in terms of impedances or admittances. As well, we can derive or calculate corresponding time-domain impulse responses from these characterizations.