Oscillation is a repeated (temporal) variation in some measure. For example, a weight hanging on a string attached to a fixed point is a pendulum, and when disturbed from its resting position it will move in an oscillating manner.
Oscillations can be structured or random. Further, structured oscillations can be periodic, which is defined as a variation that is exactly repeated after the period length. Mathematically, perhaps the most fundamental periodic oscillating function is the sine together with its dual, the cosine. Periodic signals are most often analysed by Fourier analysis which gives the amount of information on each frequency component of the signal. A sinusoidal oscillation is known in mechanics as simple harmonic motion.