pendulum

Concise Oxford English Dictionary © 2008 Oxford University Press:
pendulum /ˈpɛndjʊləm/

▶noun

• 1 a weight hung from a fixed point so that it can swing freely, especially a rod with a weighted end that regulates the mechanism of a clock.

• 2 the tendency of a situation to oscillate between extremes: the pendulum of fashion.

– derivatives
pendular adjective.

– origin C17: from L., neut. of pendulus ‘hanging down’.