Outs
The unseen cards that will improve your hand to the likely winner.
Count outs carefully. Some cards that complete your draw may also give an opponent a better hand. These "dirty outs" should be discounted.
Outs turn into percentages with the rule of 2 and 4: about 2% per out for one card, 4% per out with two cards to come.
Example
A flush draw has 9 outs: 13 cards of the suit, minus the 4 you can see.