Any rigid body that swings about a fixed point. The ideal simple pendulum consists of a bob of small mass oscillating back and forth through a small angle at the end of a string or wire of negligible mass. Such a device has a period 2π√(l/g), where l is the length of the string or wire and g is the acceleration of free fall. This type of pendulum moves with simple harmonic motion.
The compound pendulum consists of a rigid body swinging about a point within it. The period of such a pendulum is given by T = 2π√[(h2 + k2)/hg], where k is the radius of gyration about an axis through the centre of mass and h is the distance from the pivot to the centre of mass. See also Kater's pendulum.