pendulum

n. object that is suspended from a fixed point so it may swing freely

A pendulum is an object that is attached to a pivot point so 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 other words, a weight attached to a string swings back and forth.
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Noun
1. an apparatus consisting of an object mounted so that it swings freely under the influence of gravity
(hypernym) apparatus, setup
(hyponym) Foucault pendulum
(part-meronym) bob

(n.)
A body so suspended from a fixed point as to swing freely to and fro by the alternate action of gravity and momentum. It is used to regulate the movements of clockwork and other machinery.
  

An experiment first performed in 1851 to demonstrate the rotation of the earth without recourse to observing the external universe. Many public science centres have a long simple pendulum set swinging and its plane of oscillation can be observed to apparantly turn slowly; in fact its plane in space is static.