In flop games, it is the fourth community card dealt (also known as "the turn") and represents the third round of betting. In Stud games, it is the fourth card dealt to each player and represents the second round of betting.
The turn is the fourth community card in Hold'em (and is sometimes also called 'Fourth Street'). Another round of betting ensues, beginning with the active player immediately clockwise from the button. When betting action is completed for the turn round, the 'river' or 'Fifth Street' is dealt face-up on the board.