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A pendulum is a weight suspended from a pivot so that it can swing freely. When displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position. When released, the restoring force combined with the object's inertia causes it to swing back and forth, pendulum-like, at a roughly constant period. The word "pendulum" comes from Latin pendulus, meaning "hanging."

The simplest form of a pendulum is a point mass (called a bob) attached to a massless string or rod of a fixed length. This is known as an ideal pendulum. In reality, the mass is not a point, the string has mass, and there can be air resistance. These factors lead to deviations from the behavior of an ideal pendulum.

The period of a simple pendulum, which is the time it takes for one complete swing back and forth, is primarily determined by its length and the acceleration due to gravity. For small amplitudes of swing, the period is nearly independent of the amplitude. This property was famously observed by Galileo Galilei, who noted that a pendulum's swing appeared to be regular and could be used for timekeeping.

Pendulums have a wide range of applications. They are fundamental components in mechanical clocks, where their regular oscillations provide a precise measure of time. They are also used in scientific instruments like gravimeters to measure variations in the Earth's gravitational field, and in seismometers to detect earthquakes. Historically, pendulums were crucial for navigation, helping determine latitude. The concept of the pendulum is also important in physics, serving as a classic example for studying oscillatory motion and energy conservation.