The flop in poker refers to the dealing of the first three face-up cards to the board, or to those three cards themselves. This occurs in community card poker variants, particularly Texas hold 'em and Omaha hold 'em.
The three cards are dealt simultaneously following the completion of the opening round of betting. After the flop, there is a second round of betting - associated with the flop - which is then followed by the deal of a turn card.
The three cards are often dealt facedown in a stack and then that whole stack is turned faceup and quickly slid to one side to expose all the cards such that a player cannot be seen to be reacting to one particular card.
It is often considered that the flop is key to a game of Texas hold 'em, since by the conclusion of the flop a player will have seen five of the seven cards that will make up his hand at the showdown. It should be noted that while the flop marks the point at which players have a lot of information about the cards, most of the betting rounds are still to be played out.