The number of cards that would give you the winning hand.
A draw's strength is judged by turning its outs into a percentage. Example. Gutshot 4, open-ender 8, flush 9, both 15; outs × 2 is the percentage chance on the next card, so 9 outs is 18%. × 4 (seeing both remaining cards) is only for when you are all-in on the flop; above 9 outs, correct with (outs × 4) − (outs − 8). Don't count cards that would still lose (dirty outs).