Theodor Holm (`Ted') Nelson coined the terms hypertext and hypermedia. In Literary Machines he wrote:
[B]y "hypertext" I mean non-sequential writing -- text that branches and allows choices to the reader, best read at an interactive screen.
As popularly conceived, this is a series of text chunks connected by links which offer the reader different pathways.
I will not argue with this definition here, but I hope it will become clear throughout the book how much more I think hypertext can be....
Hypertext can include sequential text, and is thus the most general form of writing. Unrestricted by sequence, in hypertext, we may create new forms of writing which better reflect the structure of what we are writing about; and readers, choosing a pathway, may follow their interests or current line of thought in a way heretofore considered impossible.
The Oxford English Dictionary Additions Series includes this definition of hypertext:
Text which does not form a single sequence and which may be read in various orders; specially text and graphics ... which are interconnected in such a way that a reader of the material (as displayed at a computer terminal, etc.) can discontinue reading one document at certain points in order to consult other related matter.
I have some a list of introductory references about hypertext in which you might be interested.
There's a great example of an annotated definition of hypertext (based on the quote from Nelson above) at the University of Georgia.
This document is copyright by its author, J. Blustein.
J. Blustein <jamie@csd.uwo.ca>