A pendulum is a weight suspended from a pivot so that it can swing freely.
When the bob is displaced sideways from its resting or equilibrium position, it is subject to a restoring force.
When it is released, the restoring force causes it to oscillate about the equilibrium position. And the time for one complete cycle, a left swing, and a right swing, is called the period.
Reference Source:
[1] Wikipedia Contributors. “Pendulum.” Wikipedia, Wikimedia Foundation, 8 July 2021, en.wikipedia.org/wiki/Pendulum. Accessed 30 July 2021.