"non-sequential writing" (Ted Nelson, Literary Machines. 1974)
"the use of the computer to transcend the linear, bounded and fixed qualities of the traditional written text" (George Landow, Hypermedia and Literary Studies)
For literary purposes, Landow's definition is most useful. Extrapolating from his definition, we can say that hypertext is
non-linear (multi-linear, non-sequential)unbounded (open-ended)
un-fixed (temporary, transitory . . .)