Referring to the missing card or cards in an inside straight. When there is one missing card, it is a single Gap and when there are two, it is a double Gap. A double Gap inside straight can produce only a single straight. One the other hand a single Gap three card inside straight draw can produce up to three different straights. Often a terminal single Gap inside straight draw (a hand containing an ace) is treated as a double gap since only one straight can be drawn.
See also: Straight.