The number of cards left in the deck that will improve a player's hand to a likely winner.

For example, with the board showing {a-Clubs}{9-Diamonds}{4-Diamonds}{3-Clubs}, a player is all in with {k-Clubs}{q-Clubs} (a flush draw) versus someone with {a-Spades}{k-Spades} (top pair). The player with the flush draw has nine "outs" — namely, the nine clubs that will make a winning flush.