An inside straight draw, belly buster, or gutshot is a draw to a straight missing an inner card.
Example (Texas Hold'em):
The player has an inside straight draw with 5,7,8,9 since any six will complete it. The remaining sixes in the deck are his outs. The probability of one of them being dealt on the turn is around 8.5%. The probability that at least one six will be dealt by the river is around 16.5%.