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 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