Folks,
Non-repudiation is defined (in ISO 7498-2) as a security service
that prevents a participant in a communication from later denying his
participation in said communication (I paraphrase). Thus there are various
forms of non-repudiation that may apply to message exchanges, as Stefan
pointed out. It's not that the term "non-repudiation" is ill defined, but
rather that there are a variety of security service variants, all of which
are forms of non-repudiation. Also note that security mechanisms (that
support security services) come in varying "strengths" and thus one may
reasonably claim that "imperfect" mechanisms support a given security
service, even though we realize that there are means by which these
mechanisms may be defeated.
Steve