Backdoor

draws

A draw needing two perfect cards (turn + river) to complete.

A backdoor flush draw on the flop has only ~4% equity to make a flush by the river — runner-runner odds. Solid players still consider these in equity calculations because they add up across many hands.

See also

Drawing Dead
Having no card left in the deck that can win the hand.
Flush Draw
Four cards of the same suit, needing one more for a flush.
Gutshot
An inside straight draw — needing one specific rank (4 outs).
OESD
A straight draw with two ranks completing it (8 outs).