A pendulum is something hanging and swinging freely from a fixed point. A grandfather clock might use for timekeeping a pendulum that swings back and forth.

Pendulum comes from the Latin word pendulous, meaning "hanging down." If someone hangs a pocket watch in front of you and swings it back and forth, saying, "You're getting very sleepy," they're mimicking the movement of a pendulum. We also use the word pendulum to describe a situation moving between two poles or across a spectrum, like the balance of power in a multiparty political system.

Definitions of pendulum
  1. noun
    an apparatus consisting of an object mounted so that it swings freely under the influence of gravity
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    Foucault pendulum
    pendulum with a long wire; can swing in any direction; the change in the swing plane demonstrates the earth's rotation
    clicking pendulum indicates the exact tempo of a piece of music
    compound pendulum, physical pendulum
    pendulum consisting of an actual object allowed to rotate freely around a horizontal axis
    simple pendulum
    a hypothetical pendulum suspended by a weightless frictionless thread of constant length
    ballistic pendulum
    a physical pendulum consisting of a large mass suspended from a rod; when it is struck by a projectile its displacement is used to measure the projectile's velocity
    clock pendulum
    a physical pendulum used to regulate a clockwork mechanism
    type of:
    apparatus, setup
    equipment designed to serve a specific function
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