Gutshot

Gutshot, bellybuster and inside straight draw are all terms used to describe a hand in which the player is drawing at a single card rank to make a straight. By definition, this is at best a four-out draw (there might be fewer outs if one or more of the necessary cards is already dead).

For example, in a game of Hold 'em the player holds:

and the flop comes:

The player now needs a ten of any suit to make their gutshot straight. There are four tens unseen, so the player has four outs.

Close observers might note, however, that one of the possible tens (the ten of clubs) would cause the board to contain three clubs. This could conceivably give someone else a flush, so only three of the available tens are considered pure outs.

Double Bellybuster
A double bellybuster is a hand in which a player simultaneously has two separate gutshot straight draws but does not have an open-ended straight draw. It is sometimes called a "double inside straight draw" or "double gutshot", but the more colorful "double bellybuster" has become the more popular term.

For example, in a game of Hold 'em the player holds:

and the flop comes:

In this case, either a king or a nine, of any suit, would complete a straight. In fact, if this player makes either of their possible straights, they will have the nut straight and have an unbeatable hand.