A partial is any of the sine waves – simple periodic waves – of which a complex tone is made up. There is an important distinction between partial, harmonic, and overtone. An overtone is any partial except the fundamental. A harmonic is a partial that is an integral multiple of the fundamental. The fundamental is known as the first partial whereas harmonics or overtones are called upper partials.
Inharmonicity is a measure of how far a partial deviates from the closest ideal harmonic, typically measured in cents (a logarithmic unit used for musical intervals) for each partial.