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The overtones are a nomenclature to refer to the terms of a harmonic series of frequencies. One can define it as the component of a complex sound (i.e., sound with multiple frequencies) that has a greater tone (which has a higher frequency) that the fundamental component of the sound. Thus, the key term is the tone (fundamental) and the following terms are overtones (or higher frequency tones or secondary components).
The easiest example to understand can be seen to decompormos a musical note; for example, the key of the piano la vibrates at a frequency of 440 Hz. The second harmonic (first overtone) will vibrate at 880 Hz, the third (second overtone) in 1760 Hz.