When the frequency of an overtone is not an integer multiple of the fundamental the overtone is said to be inharmonic and is called a partial; its waveform is aperiodic.
Instruments of the percussion group, bells, and gongs in particular, have overtones which are largely inharmonic. As a result, such sounds may have indefinite or multiple pitches associated with them. (Source: Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)
See also:Fundamental, Harmonic, Spectrum