an apparatus consisting of an object mounted so that it swings freely under the influence of gravity
"The grandfather clock's pendulum swung rhythmically back and forth, keeping perfect time despite the gentle draft from the window."
A body suspended from a fixed support so that it swings freely back and forth under the influence of gravity, commonly used to regulate various devices such as clocks.
"The antique grandfather clock stopped ticking because someone accidentally knocked over its pendulum."