What is the X in X-mas?
(TDM) A type of multiplexing where two or more channels of information are transmitted over the same link by allocating a different time interval ("slot" or "slice") for the transmission of each channel. I.e. the channels take turns to use the link. Some kind of periodic synchronising signal or distinguishing identifier is usually required so that the receiver can tell which channel is which.
TDM becomes inefficient when traffic is intermittent because the time slot is still allocated even when the channel has no data to transmit. Statistical time division multiplexing was developed to overcome this problem.
Compare wavelength division multiplexing, frequency division multiplexing, code division multiplexing.