harmonic oscillator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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, as in the force exerted by a spring that is stretched or compressed or by a vibrating string on a musical instrument.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun classical mechanics A system which, when displaced from its equilibrium position, experiences a restoring force proportional to the displacement according to Hooke's law: , where is a positive constant.