1. An integral multiple of the frequency of a wave, such as a sine wave. Note 1: The first harmonic is the fundamental frequency, the second harmonic is twice the fundamental frequency, the third harmonic is three times the fundamental frequency, and so on. Note 2: A pure sine wave has no harmonics above the first harmonic, i.e., above the frequency of the wave itself. 2. In a nonsinusoidal wave, such as a square wave, an integral multiple of the fundamental frequency of the wave. Note 1: The frequencies of the harmonics, including the fundamental frequency, and their amplitudes, may be identified by a Fourier analysis of the wave. Note 2: The fundamental frequency of a periodic phenomenon, such as a square wave, is always the reciprocal of the period of the phenomenon. See single harmonic. See also amplitude,  Fourier analysis,  frequency,  fundamental frequency,  identify,  integral,  overtone,  period,  periodic,  sine wave,  sinusoidal function,  square wave,  wave.