Oscillation is a repetitive or periodic motion that regularly repeats – the object returns to a given position after a fixed time interval. The molecules in solids oscillate about their equilibrium position, electromagnetic waves such as light waves, radar and radio waves, are characterized by oscillating electric and magnetic fields.
Familiar examples of oscillation are the motion of a swinging pendulum and the motion of a spring-mass system, Figure 34.

A body that undergoes periodic motion always has a stable equilibrium position, point o of Figure 34. When it is moved away from this position and released, a force or torque comes into play to pull it back toward equilibrium. But by the time it gets there, it has picked up some kinetic energy, so it overshoots, stopping somewhere on the other side, and is again pulled back toward equilibrium.

A special kind of periodic motion occurs in mechanical systems when the force acting on an object is proportional to the position or displacement of the object relative to the equilibrium position. If the force is always directed toward the equilibrium position, the motion is called Simple Harmonic Motion (SHM).