Definition: A mechanical system consisting of a rigid body free to rotate about a pivot at a point other than its centre of mass, in the Earth's gravitational field. The equilibrium position of the system is the orientation with the centre of mass vertically below the pivot. If the orientation differs from the equilibrium position, a restoring couple acts on the body and causes oscillations. If the oscillations are small, they are harmonic.
Source: The Penguin Dictionary of Science. London: Penguin, 2009. Credo Reference. Web. 08 August 2012.