I'll weigh in here.

I'd define mental toughness as: the ability to continue to go on, even after conditions become adverse, and even after a person has been pushed past their breaking point. People who are mentally tough get up and continue to go on after they've been brought to their knees, often in tears.

Mental toughness and physical conditions (enough rest, food, etc) are directly linked. Mental toughness and a perception that participation is "worth it" are linked. Thus, mental toughness is situational and linked to the physical.

An example is, the guy who passes out while his wife is in childbirth, but goes back again knowing that he needs to support his wife, is mentally tough.

You can generalize this statement by saying "the person who crumbles or folds, but continues to go on in spite of the continued precense of the stress that caused them to fold, because they need to do something important, is mentally tough."