A hand that can only win the pot if a single card in the deck is dealt.
Example: Hero has AsAc. Villain has QcJc. The board is KcTcAhAd. As Hero has 4 aces including the ace of clubs, the only card in the deck that can win the hand for Villain is the 9c. If the 9c comes on the river, Villain has hit a one outer.