Typically refers to the altering of a carrier wave in order to transmit a data signal (text, voice, audio, video, etc.) from one location to another via a discrete channel (see carrier). At the receiving end, a tuner latches onto the particular carrier frequency and a demodulator circuit isolates the data from the carrier.

In analog telephony, modulation also refers to the altering of a DC current in order to transmit voice electronically. In such a conversation, both parties are modulating the current simultaneously.

Amplitude, Frequency and Phase
There are three basic types of modulation: amplitude modulation (AM) varies the voltage; frequency modulation (FM) varies the frequency, and phase modulation (PM) varies the angle of the wave (see amplitude modulation, frequency modulation and phase modulation). In quadrature amplitude modulation (QAM), both the amplitude and phase of the carrier are modified (see QAM).