Definition of nit
A nit is a tight player, a player who plays very few hands. The term "nit" has a pejorative connotation. A good style of winning poker is a TAG (tight + agressif) style, but nit is synonymous with a player who doesn't play enough hands. A player who 3bet with AA, KK, QQ and AK only, a player who pushes all in with aces only, a player who often folds preflop, a player who doesn't bet enough, who fold too much is a nit.
How do you play against a nit?
Against a nit, first of all, you have to give credit to your 3bets and even more to your 4bets. When the nit player raises or re-raises, he usually has a hand. When a nit-check, he won't be very strong (unless he check-raises). You will be able to bluff the nits more often than any other type of player. Play the nits in position, when he checks, bet, when he bets, fold. It's not rocket science. The biggest drawback of the nit player is that it is predictable, which will make it easy to read. As long as we know roughly what hand he has, he won't have much success against us.