In time-domain filtering each element of the original signal is replaced by a sequence of elements, proportional in amplitude to the original signal but spaced in time; the sum (assuming linear fitering) of these sequences forms the new signal. In transform-domain filtering the elements of the original signal are not those of its amplitude but rather of its components under, for example, Fourier analysis or Walsh analysis; they are then spaced not in time but in frequency or sequency respectively. Many other transforms are also used.
Filtering, both in the time domain and in various transform domains, is of great importance in multiplexing. A simple but very common example of filtering in the frequency (Fourier) domain is the use of passive resonant circuits, RC circuits, or active filters to effect low-pass, band-pass, high-pass, and band-stop functions; these are much used, e.g. in data transmission lines and modems.