An
overtone is a natural resonance or vibration frequency of a system. Systems described by overtones are often sound systems, for example, blown pipes or plucked strings. If such a system is excited, a number of sound frequencies may be produced, including a fundamental tone of given frequency. An integer multiple of the fundamental frequency is called a harmonic.
The second overtone is not the second harmonic.
(See related link "Calculations of Harmonics and Overtones from Fundamental Frequency")