A pendulum is an object that is attached to a pivot point so that it can swing freely. This object is subject to a restoring force that will accelerate it toward an equilibrium position. When the pendulum is displaced from its place of rest, the restoring force will cause the pendulum to oscillate about the equilibrium position. In English a weight attached to a string swings back and fourth.
A basic example is the simple gravity pendulum or bob pendulum. This a weight (or bob) on the end of a massless string, which, when given an initial push, will swing back and forth under the influence of gravity over its central (lowest) point.