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 winning poker style is TAG (tight + aggressive), but nit is synonymous with a player who does not play enough hands. A player who 3-bets 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 does not bet enough, who folds too much, is a nit.
How do you play against a nit?
Against a nit, first of all, you have to give credit to his 3bets and even more to his 4bets. When the nit player raises or re-raises, he usually has a hand. When a nit checks, he won't be very strong (unless he checks-raises). You will be able to bluff nits more often than another type of player. Play the nits in position, when they check, bet, when they bet, fold. It's not rocket science. The biggest flaw of the nit player is that he is predictable, which will make him easy to read... As long as we know roughly what hand he has, he won't have much success against us.