The frequencies of overtones in a normal harmonic series are directly related to integer multiples of the fundamental frequency. In contrast, inharmonicity is the extent to which the frequencies of individual overtones deviate from integer multiples of the fundamental frequency.
In a musical context, percussion instruments such as chimes or cymbals often create strong inharmonic overtones, but the effect is also present in many other instruments, most notably the piano, and is directly responsible for the concept of stretch-tuning.