QAM
Quadrature Amplitude Modulation: A hybrid amplitude/phase modulation technique that allows the transmission of four bits of information during a signaling interval, and requires less bandwidth than normal amplitude or phase modulation techniques.
QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinewave) in response to a data signal. In the case of QAM, the amplitude of two waves, 90 degrees out-of-phase with each other (in quadrature) are changed (modulated or keyed) to represent the data signal.