Dictionary.net

A musical sound incapable of resolution, and resulting from a simple vibration. Suppose a stretched string to make any number (say 264) of complete vibrations a second ; if its length were reduced to a half, a third, a fourth, etc., other things remaining the same, it will make 528, 792, 1056, etc., vibrations a second : the tone corresponding to the 264 vibrations is the Fundamental Tone of the string ; those corresponding to the others (viz. the octave, the fifth above the octave, the second octave, etc.) are the Harmonics, i.e. the acute harmonics, or harmonic overtones, of the fundamental tone.