Harmonic
A harmonic of a wave is the part of a signal's frequency that is a whole multiple (an integer) of the fundamental frequency. The fundamental frequency is the lowest frequency of a periodic waveform. If f is the fundamental frequency (that is, 1f), then the harmonics have frequencies 2f, 3f, 4f..., and so on. The second harmonic (2f) has a frequency twice that of the fundamental frequency, the third harmonic three times, and so on. This concept applies to wave motions of any form of energy, but is perhaps most often related to acoustics.