A physical system in which some value oscillates above and below a mean value at one or more characteristic frequencies. Such systems often arise when a contrary force results from displacement from a force-neutral position, and gets stronger in proportion to the amount of displacement. For example, pulling or pushing the end of a spring from its rest position results in a force pushing back toward the rest position. Letting the spring go from a position of tension results in harmonic motion of the spring; the spring is now a harmonic oscillator. Other examples include a swinging pendulum, a vibrating violin string, or an electronic circuit that produces radio waves.