What is an oscillation?

A repeated back and forth motion (i.e alternate "to and fro" or "backwards and forward" motion) of a body about its mean position is called as oscillation or vibration.When an object moves back and forth repeatedly we can say that it is making oscillations. Eg: When a swing moves back and forth we say that the swing is oscillating. Motion of a swing and simple pendulum are examples of oscillations.

i.e A body is said to complete one oscillation when the body goes to one side of its mean position, comes back to that position and then goes to other side and again returns to the same mean position.