inharmonicity 

The degree to which the frequencies of overtones (known as partials, partial tones, or harmonics) depart from whole multiples of the fundamental frequency.

in stringed instruments such as the piano, violin, and guitar, the overtones are close to—or in some cases, quite exactly—whole number multiples of the fundamental frequency. Any departure from this ideal harmonic series is known as inharmonicity. The less elastic the strings are (that is, the shorter, thicker, and stiffer they are), the more inharmonicity they exhibit.

inharmonicity by noduesp July 20, 2011