Inharmonicity is a measure, usually in terms of cents, of the deviation of a partial from the nearest ideal harmonic. Partials are all of the components of a waveform, including the fundamental frequency and the overtones. A harmonic is an overtone that is a perfect integer multiple of the fundamental frequency. An inharmonic partial is any partial that does not match an ideal harmonic.
Not only is some extent of inharmonicity unavoidable in traditional musical instruments, but it is also not necessarily undesirable. In fact, it subtly contributes to the unique timbre of instruments, and may produce the "warmth" characteristics of conventional (mechanical) pianos as compared with their electronic counterparts. Thus it is purposely incorporated into some electronic instruments as a means of increasing the realism of their sound output.