The Free On-line Dictionary of Computing (30 December 2018):
1. (Or "multiple access") Combining several
signals for transmission on some shared medium (e.g. a
telephone wire). The signals are combined at the transmitter
by a multiplexor (a "mux") and split up at the receiver by a
demultiplexor. The communications channel may be shared
between the independent signals in one of several different
ways: time division multiplexing, frequency division
multiplexing, or code division multiplexing.
If the inputs take turns to use the output channel (time
division multiplexing) then the output bandwidth need be no
greater than the maximum bandwidth of any input.
If many inputs may be active simultaneously then the output
bandwidth must be at least as great as the total bandwidth of
all simultaneously active inputs. In this case the
multiplexor is also known as a concentrator.
2. Writing multiple logical copies of datafiles. Placing the copies on totally separate paths to
mirrored devices greatly reduces the probability of all
copies being corrupt. Multiplexing differs from mirroring in
that mirroring takes one data file and copies it to many
devices, thus making it possible to copy a corrupt file many
times. Multiplexing writes the data files to many places
simultaneously; there is no "original" data file.