An inharmonic partial is any partial that does not match an ideal harmonic. Ideal harmonics are overtones that are whole number multiples of the fundamental frequency. Partials are all of the components of a waveform; that is, the fundamental frequency plus all the overtones. A harmonic is an overtone that is a perfect integer multiple of the fundamental frequency. Inharmonicity is a measure, usually in terms of cents, of the deviation of a partial from the nearest ideal harmonic.
Not only is some extent of inharmonicity unavoidable in traditional musical instruments, but it can also be desirable, because it contributes subtly 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.