A pendulum is usually a crystal that is on the end of a string, chain, or rope. Crystals are a great choice for using as a pendulum as they already possess a powerful frequency and a deep dose of spirituality. It’s called a pendulum because of the motion it uses when offering guidance - that is, it pendulates in different directions according to the answer.
Pendulums are used to answers our questions, mostly with "yes" or "no" responses, and can give us better insights to certain circumstances.