Definition of nut flush draw
"Nut flush draw" means "maximum colour draw". If you have A5 in spades and the flop is 4-8-K with two spades on the flop, you have a nut flush draw. This is the maximum colour draw. If you have the ace in hand, your hand has good equity, even against pairs. To use our previous example, if you have A5 in spades on this board against a hand like QQ, you have 47% equity on the flop, without even having a pair.
How should you play your nut flush draw?
A nut flush draw is stronger on the flop than on the turn. Already, this is a good place to start thinking about how to play them. On the flop, if you think you have folding equity, it's best to play your hand aggressively. Let's stick with our example. One player has A5 in spades on 4-8-K and the other player has QQ. If the player with QQ bets the flop and the player with A5 checks-raises, he puts the player with QQ in a difficult situation. So, by making a check-raise, the player with A5 wins the hand part of the time and if he gets called, he can continue to bet on the turn with folding equity. And he is left with the outs (cards that can help him), 9 cards in spades plus the aces that will give him top pair.