After reading any number of different sources, I still don’t know the answer to:
Is it illegal under copyright law for me to display the definition of a word (all 15 meanings) on a particular web page that I am writing, even if I give full reference?
It depends on the word. But, I’d say, in most cases, no.
If the word was registered or trademarked though, then that is where things get hairy. If your definitions of those registered or trademarked words were slanderous or defaming (even if you listed a source), you (and your source) could be in trouble.
I assume you’re lifting the definition from a dictionary. If you reference the dictionary, you’re probably safe under fair use. It is only a small portion of the entire dictionary, which gives you an edge in court, so much so that it’d be unlikely the dictionary would push the issue.