The best possible flush for a given situation. It must not
be the nut hand overall. For example, if the board in hold'em is 5♥
8♥ 3♠ 5♦
J♥ and you have A♥
2♥, your ace-high flush is the nut flush but your opponent
could hold 5♠ 8♠ for a full house.
See also straight flush and