harmonic motion

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

• noun A periodic vibration, as of a pendulum, in which the motions are symmetrical about a region of equilibrium. Such a vibration may have only one frequency and amplitude or may be a combination of two or more components.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a periodic motion in which the displacement is either symmetrical about a point or is the sum of such motions