In communications and broadcasting, a process by which one characteristic of a signal or waveform is varied in accordance with another characteristic. Modulation is performed so as to transform a an originating signal (such as from a computer) into one that is compatible with the transmission medium and communications equipment. Upon reception, the modulated signal needs to underdo demodulation. (Hence the term modem, which is a device that performs both modulation and demodulation functions, is a contraction of "'mo'dulator/'dem'odulator.")
In broadcasting, modulation refers to a similar process of overlaying an electrical signal (derived from the sound wave produced by, say, a radio station) onto a carrier wave. See Amplitude Modulation and Frequency Modulation.
See also Pulse Code Modulation.