A
poker hand where you hold four consecutive cards of a
straight, missing one on the end. For example 5678 and 6789 are open-ended straight draws, whereas 5689 is an
inside straight draw.
Open-ended straight draws are much better hands than inside straights, because twice as many cards will complete your straight. In the above examples, the first hand needs either a 4 or a 9 to make a straight. The seconds makes on a 5 or a ten. But the last one only hits on a 7. An open-ended straight has a slightly worse chance to hit than a four card flush, if no other cards are known to be out of play.
The term is also used generically to describe any hand that needs two cards to hit, even when the cards in the hand are not consecutive and the needed cards aren't at the ends. A common example is when a hand contains two distinct inside straight draws.
In Texas Hold-em an open-ended straight draw will hit 8 out of 47 times (five of the 52 cards are known and 8 will make the straight) on the turn. If not, it will river an additional 8 out of 46 times (one more card is known, but the 8 cards that make the straight are still in the deck). By solving the conditional probability, you can expect about 1/3 of your flopped open-ended straight draws to hit.