Oscillation is a repeated (temporal) variation in some measure. For example, a weight hanging on a string attached to a permanent point is a pendlum, and when deviated from its resting position it will move in an oscillating manner.
Oscillations can be strctured or random. Further, structured oscillations can be periodic, which is defined as a variation that is exactly repeated after the period length. Mathematically, perhapse 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.
Typical oscillating physical systems are, for example, a pendulm, the moon orbiting around the earth, electrical oscillation in a radio reciever or transmitter, sound waves and waves on the surface of water.