I always think of "fairness" (regardless of format) as a measure of how closely a deck sticks to core Magic gameplay: advance your mana by approximately one per turn, play a small number of spells per turn, answer your opponent's threats, attack with creatures, gradually lower your opponent's life total. On that scale a deck is "unfair" to the extent that it seeks to bypass that gameplay and play a different way. So for example, Shops is an unfair deck because it advances its mana way too quickly, Storm is a more unfair deck than that because it completely refuses to even engage with what its opponent is doing beyond answering their disruptive plays, and Dredge is the least fair of all because it sidesteps the basic rules about how cards are drawn and played.
This a relative term, though. In the grand scheme of all things Magic, I don't think there is a viable Vintage deck that is truly fair... decks that are commonly described as fair do still have game-breaking cards like moxes and lotus and stuff like Monastery Mentor or whatever, and I would also say that while countering spells is generally considered a fair tactic, playing a card that just stops your opponent from playing most of their spells (i.e. Lavinia) is probably not. I would still say that it's a useful term even if it isn't perfectly descriptive though, since it does refer to a class of decks that is sometimes worth talking about as a group, and I think people generally know what you mean when you say it, barring a few edge cases.