Outs
The cards remaining in the deck that turn a hand into the winner.
Outs are the building block of the rule of 2 and 4, the standard shortcut for estimating equity at the table: roughly outs times four with two cards to come, or outs times two with one card left. Not every out is clean. A card that completes a hand can complete something better for an opponent at the same time, such as a flush card that also pairs the board into a full house for someone else, and a dirty out like that should be discounted or dropped from the count entirely.