- An oscillator which produces harmonics of a fundamental frequency.
- A crystal oscillator which produces harmonics of the natural frequency of vibration its piezoelectric crystal.
- A physical system, such as a pendulum, which exhibits harmonic motion. Also called simple harmonic oscillator.
- For an oscillating system, an equilibrium-restoring force which is proportional to the displacement from the equilibrium position. This contrasts with an anharmonic oscillator, in which the forces restoring the equilibrium position are not proportional to the displacement from it.