According to Gerard Gennette, French literary critic and
author of
Palimpsests: Literature in the Second Degree, a hypertext is any text that is related to an earlier text (which he calls the
hypotext) "in a manner that is not that of commentary".
Almost any text is a hypertext, related to other works via hypertextuality, or more generally intertextuality, "grafted", as Gennette says, onto these other works, because no text is completely isolated or original. Every text is recycled culture.
The recent and technological meaning for "hypertext" is really about a method for making obvious the linkages between texts. The links have always been there, since there has been writing (or perhaps before, with oral tradition), but the technology of the World Wide Web and other devices have simplified the navigation of these relationships.