This term came into widespread use with the emergence of electronic documents, especially after the Web was created. Since then “multi-channel” has grown to mean any number of channels and even an unknown, n + 1, number of channels given the proliferation of devices and applications.
“Omnichannel” or omni-channel publishing became a popular concept among marketers to refer to “all the channels”, including physical and digital, to a customer, and the desire to synchronize all channels and touch points for a good customer experience. Marketers are of course hopeful by nature.
Also see single source publishing, multipurpose publishing, SGML, and XML.