The Probert Encyclopaedia of Music

HARMONIC

A harmonic is a musical note produced by a number of vibrations which is a multiple of the number producing some other; an overtone.
Harmonics are secondary and less distinct tones which accompany any principal, and apparently simple, tone, as the octave, the twelfth, the fifteenth, and the seventeenth. The name is also applied to the artificial tones produced by a string or column of air, when the impulse given to it suffices only to make a part of the string or column vibrate.
Research Harmonic