Wikipedia: Quadrature amplitude modulation

Quadrature Amplitude Modulation (QAM) is the encoding of information into a carrier wave by variation of the amplitude of both the carrier wave and a 'quadrature' carrier that is 90° out of phase with the main carrier in accordance with two input signals.

Alternately, this can be regarded (using complex number notation) as simple amplitude modulation of a complex-valued carrier wave by a single complex-valued signal.

Phase modulation can also be regarded as a special case of quadrature amplitude modulation, where the amplitude of the modulating signal is constant, with only the phase varying. This can also be extended to frequency modulation, as this can be regarded as a special case of phase modulation.

QAM is used extensively in modems, and other forms of digital communication over analog channels.

In these applications, the modulating signal is generally quantised in both its in-phase and 90° components. The set of possible combinations of amplitudes, as shown on an x-y plot, is a pattern of dots known as a QAM constellation.