Maniac
A maniac is a very loose and very aggressive player who bets and raises with many hands, often when it is not a good idea. He puts lots of chips into the pot and makes the game very wild.
The simple explanation
A maniac plays many hands and raises a lot. This makes games swingy. The best way to beat a maniac is to be patient, wait for strong hands and let him make bets for you.
A closer look
Against a maniac, the standard adjustments are to call down lighter with decent hands, to avoid bluffing him, since he may not fold, and to value bet thinly. You can also trap him by checking strong hands and letting him bet. The danger is being drawn into big pots with marginal hands. Maniacs can be hard to play against emotionally, so stay disciplined and keep your stack in mind.
A practical example
A player raises 15 hands in a row and bets large on every street. The table calls him a maniac.
Full explanation
A maniac is a very loose and very aggressive player who bets and raises with many hands, often when it is not a good idea. He puts lots of chips into the pot and makes the game very wild.
A maniac is an extremely loose and aggressive player.
He bets and raises with a wide range, often with weak hands.
Against him, play strong hands and let him do the betting.