An oscillator that does not obey Hooke's law. This law is an idealized expression that assumes that a system displaced from equilibrium responds with a restoring force whose magnitude is proportional to the displacement. The use of Hooke's law results in a linear equation of motion that fails to describe many properties of the real world. Nature demonstrates two fundamentally different forms of nonlinearity, which may be called elastic anharmonicity and damping anharmonicity. To understand their difference, and the nature of a harmonic oscillator, it is necessary to understand potential functions.