In classical mechanics, anharmonicity is the deviation of a system from being a harmonic oscillator. An oscillator that is not oscillating in simple harmonic motion is known as an anharmonic oscillator where the system can be approximated to a harmonic oscillator and the anharmonicity can be calculated using perturbation theory. If the anharmonicity is large then other numerical techniques have to be used.

As a result, oscillations with frequencies and etc., where is the fundamental frequency of the oscillator, appear. Furthermore, the frequency deviates from the frequency of the harmonic oscillations. As a first approximation, the frequency shift is proportional to the square of the oscillation amplitude :

In a system of oscillators with natural frequencies, ... anharmonicity results in additional oscillations with frequencies .

Anharmonicity also modifies the profile of the resonance curve, leading to interesting phenomena such as the foldover effect and superharmonic resonance.

Read more about Anharmonicity:  General Principle, Examples in Physics, Potential Energy From Period of Oscillations