A card that can no longer come out of the deck because it is already visible.
In Stud, other players' up cards and cards shown by folded players are dead cards, and if cards you need are showing, your outs shrink by that many. Example. With a 3-flush and 3 cards of your suit visible, 10 − 3 = 7 remain. At an 8-handed table (7 other door cards) the completion rate drops from 23% to 12%; at a 6-handed table (5 door cards), from 22% to 11%.