Consider that the bob of a simple pendulum has moved from its mean position to one of its extreme positions and then again to mean position from where it moved to the other extreme position and then back to the mean position. The bob of the simple pendulum has now completed:
One oscillation
If bob of the simple pendulum moved from its mean position to one of its extreme positions and then again to its mean position from where it moved to the other extreme position and then back to the mean position it would have completed one oscillation.
In other words, one complete cycle of repetitive to and fro motion, from one extreme (maximum or minimum) to another (minimum or maximum) and back is called one oscillation.