Harmonic timbre
A timbre is harmonic when its frequency spectrum has the property that all overtones are exact multiples of the lowest frequency. This causes the sound wave to be periodic (where the frequency of the sound wave is the frequency of the lowest note in the spectrum) and causes just intervals to be concordant through the alignment of partials. Examples of harmonic timbres include those of most instruments, the human voice, and most synthesized tones (like saw, square, and triangle waves).
A timbre can be said to be nearly harmonic if its overtones are approximately equal to integer multiples of the fundamental frequency. Examples include most plucked string instruments (like cellos played pizzicato, guitars, and pianos).
A timbre is inharmonic if its overtones deviate significantly from integer multiples of the fundamental. Examples include metallophones (like those used in gamelan) and drums.