Definition: In Texas Hold'em, if you have 3 cards to a straight or a flush and get both the cards you need, it is said you've made a "backdoor" straight or flush.
In other words, if you need 2 cards to make your hand, both the turn and the river, you are hoping to make a backdoor flush or straight.
Also Known As: Runner-runner, runner runner, Backdoor flush, backdoor straight
Alternate Spellings: back door
I stayed in because I had a pair, but little did I know I would get runner-runner spades to make a backdoor flush!