The nut flush is the best possible flush given the current layout of cards. It is not necessarily the nuts, but there is certainly no stronger flush.

While in most cases the player holding the ace of the suit has the nut flush, this is not always the case. For example, in hold'em if the ace is one of the community cards, the player holding a king, and having a flush, would have the best possible flush. Similarly, in a game of seven card stud if the ace and king are exposed in other hands, where a flush is not possible, a player with a queen high flush would have the nut flush.