An harmonic is a signal whose frequency is an integral multiple of the frequency of some reference signal. For a signal whose main frequency is f, the second harmonic has a frequency 2f, the third harmonic has a frequency of 3f, and so on. Signals occurring at frequencies of 2f, 4f, 6f, etc. are called even harmonics; the signals at frequencies of 3f, 5f, 7f, etc. are called odd harmonics.