modulation
n. changes in some parameter of a waveform (e.g., amplitude, frequency, phase) so that the information contained by the variations of this parameter can be transmitted by the wave, which is known as the carrier wave. Amplitude modulation (AM) refers to changes in amplitude that are relatively slow compared to the usually sinusoidal variations in the carrier. In frequency modulation (FM), the frequency of the carrier is varied, but its amplitude remains constant. In phase modulation, the relative phase of the carrier wave is varied in accordance with the amplitude of the signal variations. For most modulated waveforms, the frequency of modulation is much less than the frequency components in the carrier.