Definition of bouton
The bouton is the tablet on the table of poker on which it is written "DEALER". It is the tablet that identifies who is the dealer in the hand. In a casino, this term is symbolic since usually (unless you are in a friendly game), it is not the player who will play the role of dealer, but a designated dealer who does not play. When we talk about the bouton, we are talking about the player who has that bouton and is therefore the dealer (the player in position).
Additional details about the bouton
On each hand, the bouton will circulate clockwise. If a player was bouton in one hand, the next hand, the bouton will move left and the next player will move left. The bouton is important because it determines the player in position (the player to speak last out of all streets after the flop). The bouton also determines the blindss. The player to the left of the bouton will be the small blind and the player to the left of the small blind will be the big blind. When we talk about the bouton, we are generally not talking about the bouton with "DEALER" written on it, but about the player in position, the dealer. In short, the bouton is a position on a poker table in the same way as the small or big blind.