A periodic function satisfying certain mathematical conditions can be expanded in a series.
This mathematical theorem has fundamental practical importance in Telecommunications and explains how any practical periodic wave shape can be as the sum of an infinite number (a series) of sine wave components.
The amplitude, frequency and phase of each component are in a precise relationship to that of the first term of the series. The first component (term) of series is called the Fundamental, while the subsequent ones are the Harmonics of the fundamental.
The word Harmonics is clearly taken from Acoustics and we matches the fact that each component has a frequency which is an integer multiple of the fundamental (F, 2F, 3F, 4F....)
The amplitude of each term is lower for terms a higher number, so that while considering of order given signal (the the ensemble of the frequency components of a so-called SPECTRUM), it is not necessary to consider infinite harmonics since the amplitude (and importance) of these becomes quickly negligible practice a Fourier's analysis of a signal is never pushed beyond the 7th term, n while stopping at the 3rd is satisfactory for most applications.