Definition of backdoor
The term "backdoor" in poker means that a player's hand has improved with the help of two cards, the turn and the river. If we have in our hand, for example, two cards in spades on a flop that contains two spades, we would say we have a draw. But if the flop has only one spade, we have a "backdoor" draw since we need two cards (in this case a spade on the turn and the river).
Examples of a backdoor print
Player A receives his cards preflop and finds AA. Player B receives KK. The flop falls A-2-8 rainbow. In this situation, Player B will need two cards to help him. If the turn is a king and the river is also a king, the player is said to have made a backdoor four of a kind. Another less extreme example would be this. Player A and B face off on an A-T-2 flop. Player A has AK while Player B has a set of 10s (pair of 10s in hand). Here, Player A will need two cards to improve his hand. His equity will usually be low in this type of situation, about 5%. If the turn is an A and the river is a K, Player A is said to have won the hand on a backdoor draw (also called a runner-runner).