Real Dictionary

pendulum
countable noun: The pendulum of a clock is a rod with a weight at the end which swings from side to side in order to make the clock work.
singular noun: You can use the idea of a pendulum and the way it swings regularly as a way of talking about regular changes in a situation or in people's opinions.

pendulum in American English
a body hung from a fixed point so as to swing freely to and fro under the combined forces of gravity and momentum: often used in regulating the movement of clocks
noun: a body so suspended from a fixed point as to move to and fro by the action of gravity and acquired momentum

pendulum in British English
noun: a body mounted so that it can swing freely under the influence of gravity. It is either a bob hung on a light thread ( simple pendulum) or a more complex structure ( compound pendulum)