Foul
A foul hand is a hand that cannot be played for one of several reasons. A player with a foul hand may not claim any part of the pot.
The simple explanation
If a player has the wrong number of cards, the dealer declares his hand foul, and he loses the right to win the pot.
A closer look
A hand is most often foul because it has too many or too few cards. The exact rules are decided by the room. For example, in Hold'em, a player who ends up with three hole cards after the flop is declared to have a foul hand. In that case the dealer or floorman makes the decision. A foul deck, with the wrong cards, is a related problem.
A practical example
A player ends up with three cards after the flop, so the dealer declares his hand foul.
Full explanation
A foul hand is a hand that cannot be played for one of several reasons. A player with a foul hand may not claim any part of the pot.
A foul hand cannot be played.
It cannot claim any part of the pot.
A common cause is the wrong number of cards.
The floor decides unusual cases.