From Glossary of Meteorology
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  1. A swinging, as of a pendulum.

    Often applied to periodic motion or variation in time of any quantity, although may mean any more or less regular variation between fixed bounds. Fluctuation is more suggestive of irregular variation. Vibration is a near synonym except that oscillation is applied to variations in space as well as time. The amplitude of a damped oscillation steadily decreases. Oscillations are said to be forced or free according to whether the oscillating system is or is not acted upon by an external force, although what constitutes such a force is a matter of convention. An ordinary pendulum is acted on by the external force of gravity, and yet the pendulum probably would be described as undergoing free oscillation.

  2. As used by Sir Gilbert Walker, a single number, empirically derived, that represents the distribution of pressure and temperature over a wide ocean area.

    Basically, the process is one of weighting pressure and temperature values for selected island and coastal stations, and algebraically combining them. These numbers were originally employed in correlations with single station values. Three such “oscillations” were derived: the North Atlantic oscillation; the North Pacific oscillation; and the Southern Oscillation.