Balance gets talked about more than it gets defined. One definition I've encountered that I find useful is: balance is maximizing the meaningful and
viable choices available to the player. In a sense, balance is just a game being honest with you about the the choices it offers. In an imbalanced game, good, viable, marginal, terrible, and wildly broken options will be equally weighted, as if they were all of equal worth in the context of play. In a balanced game, equally weighted choices will be equally viable, superior choices will carry a heavier weight - they'll be more costly in some sense, you get fewer of them in some way, or whatever.
Fair isn't an obscure term, and I don't think it's being used in an unusual way, here... it strikes me as a very low and easy to meet bar, tho.
Like, in the context of an RPG, no matter how badly, say, classes are balanced, as long as every player can choose the one OP class that obviates all others, it's
fair - and, y'know, so what?

If an imbalanced game goes out of its way to disguise or even promote bad choices (a "trap"), I suppose it might cross the line and be arguably unfair, as well, since there could be whole swaths of players whose preferences predispose them to the trap choices.