I'm 47 and have coached high school players, etc. for several years. There are a number of books out there that our pals here have enjoyed, so I'll offer the one that's been the stand-out for me; Vic Braden's Mental Tennis. It's full of stuff I wish I'd learned when I was a kid and more than just one or two of my pals (and students) have really enjoyed it, too.
Great topic for a paper, by the way! Mental toughness is an intangible that definitely boils down to more than just one thing, but is also as essential to success in tennis as a decent set of strokes. The actual definition may require a paper or even a book to write it all down.
It seems to me that mental toughness is very much about consciously deciding what to pay attention to out there. It's easy to see a player mentally fall apart when they decide to be distracted by something that could easily be ignored by most of us. That mental check-out isn't hard to spot if you watch for it.
The "tough" player is the one who stays inside his or her own head and doesn't get mentally derailed, even when an opponent gets confrontational over line calls, etc. That toughness could mean having the awareness to disregard potential distractions, either external (opponent, wind) or internal (fatigue, etc.).