A Simple Pendulum is a system where a particle of mass is attached to an unstretched and massless string from a rigid support.
The mass hanged along the string is known as bob.
When the bob starts moving, first, it goes to the extreme position, it comes back, and again goes to the extreme position of another end.
After oscillating from one end to another end gradually it comes to rest and this position is known as the Mean positionor the equilibrium position of the oscillating object.
The oscillatory motion of a Simple pendulum is defined as the periodic to and fro motion of the pendulum.
When the to and fro motion of the bob starts from one end and comes back to the same position, it is known as one oscillation of a pendulum.