Furiten, meaning sacred discard, is a hand status. A player in tenpai is furiten if he could win on an earlier self-discarded tile, or if he has recently ignored to win by ron. Players in furiten may still win by tsumo, but not by ron.
A player with a tenpai hand is furiten if at least one of the conditions below applies.
Players in furiten may not win by ron, even if they have a yaku. Declaring ron while in furiten is penalized with a chombo payment.
Players in furiten may still win by tsumo.
A player is furiten if in his discard pile, there is a copy of any of his hand-completing tiles. It does not matter whether this tile would have provided a yaku or not. Tiles turned sideways within open melds are considered part of their discarder's discard pile.
As long as the player has not declared riichi, they can get out of furiten by altering their hand and waiting on other tiles.
This hand has three different tiles to wait on. If the player has a 2-pin in his own discard pile, he is furiten and may not win by ron on any tile. Even if a 5-pin or 8-pin gets discarded by an opponent, he may not call ron.
A riichi declarer is furiten when he has missed a chance to call ron against a winning tile after his riichi declaration. The ignored winning tile may have been a discarded tile, or a tile used to extend a minkou to a shominkan.
The player will remain furiten for the rest of the hand. He cannot call ron on any tile, even if it is a different tile than the ignored one. This rule requires knowledge of all own possible waits.
A player who has not declared riichi may also ignore a hand-completing tile and not call ron on it. Again, this may have been a discarded tile or the tile for a shominkan.
The player is then temporarily furiten. This lasts until his next own draw. Some rules additionally cancel temporary furiten on any tile call, i.e., chii, pon, or kan, by any player.
The furiten rule does not consider yaku. Sometimes, a hand can be completed with several tiles, but only some of them provide a yaku, others would produce a yakuless hand. If a yakuless tile appears, it must be ignored, because a winning hand must contain a yaku. The player must remain temporarily furiten.
Furiten is often used to defend against opponents. By discarding tiles that are also visible in an opponent's discard pile, a player can avoid a ron call by that opponent.