In telecommunications, multiplexing (MUXing) is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). The reverse of this is known as Inverse multiplexing (see inverse multiplexer).
In electrical communications, the two basic forms of multiplexing are time-division multiplexing (TDM) and frequency-division multiplexing (FDM). In optical communications, the analog of FDM is referred to as wavelength division multiplexing (WDM). See also code division multiple access
Source: from Federal Standard 1037C
In 1910 George O. Squier (1863-1934) invented the principal of multiplexing using a carrier frequency to combine multiple telephone signals on one telephone line.
In spectroscopy the term is used in a related sense to indicate that the experiment is performed with a mixture of frequencies at once and their respective response unravelled afterwards using the Fourier transform principle
When encoding video, multiplexing refers to the process of interleaving audio and video into one coherent stream. The reverse process, creating two separate streams, is called demultiplexing , often abbreviated as demuxing.