Four cards of a straight with a card in the middle missing. (ex: If you have a 9 and an 8 and the flop contains a 7, then you need a 6 in the turn or river. If a 6 comes, then you have made the gut shot straight draw.)
To draw and complete a straight with one of the middle cards missing. Ex: If you have a 10 and a 6 as your down cards and the flop contains a 7 and a 9, then you need an 8 in the turn or river card to complete the straight.
Another term for an Inside Straight Draw.