A condition in which a system is driven at a frequency equal to one of its natural frequencies. Resonance is characterized by high amplitude in relation to the driving (exciting) function. Also, at resonance the driving function (force, voltage, etc.) is exactly opposed by the system's dissipative function (mechanical loss, electrical loss, etc.).

For a piezoelectric transducer, "resonance" is sometimes used to indicate series resonance or short circuit resonance. To avoid confusion, the use of "resonance" in this context should be avoided.