Some words are ambiguous enough that a dictionary cannot fully describe. I think this is one of those. Second, wikipedia is a horrible source to trust for anything debatable.
It's debatable that the definition of "zero day exploit" is debatable. Do you also mistrust what Wikipedia has to say about immunization, global warming, homeopathy and evolution?
He asked "Do you have a source for that definition?" and you said you didn't have a source as reliable as Wikipedia, then attacked the reliability of wikipedia, which leaves your source in doubt. So what IS you source?
That's what he asked in the first place, and now that you're hopefully done casting doubt upon his source, you still haven't answered what your source is yet, except to say that it's less reliable than "a horrible source to trust for anything debatable". So please give us a link to your source, so we can see it ourselves.