A yaku is a certain pattern in a mahjong hand, or a special condition under which a win is declared.
A player requires at least one yaku to declare a win.
The game rules define lots of different yaku, worth a different amount of han based on their difficulty. A player can incorporate several different yaku in the same hand and add their han values for high-scoring hands.
Dora are not considered yaku, even though they provide han. A hand having no yaku may not win, even if it has multiple dora.
This hand has a complete sequence of 1 through 9 of a single suit. As a note, the sequence of 1-9 are actually three groups of sequences numbered 123, 456, and 789.
The hand is composed of nothing but all terminals and honors. † This hand may be considered as 4 han, because it is impossible to score this hand without either chii toitsu or toi toi.
The hand contains two sets of 3 dragon tiles and a pair of the third dragon tiles. † This hand may be considered as 4 han, because it is impossible to score this hand without two sets of yakuhai, from the two sets of dragons.
This hand is composed entirely of tiles from only one of the three suits. It is the only yaku set at 6 han, where the number drops to 5 han when opened.
This hand has one of each of the 13 different terminal and honor tiles plus one extra terminal or honour tile. Some rules may allow double yakuman for a 13-wait set.
This hand is composed of four sets of closed triplets. When this hand has a shanpon pattern and the win is via ron, then it would not be counted as suu ankou.
The non-dealer hand is a winning hand after the first draw.
Special case
This yaku does not specifically deal with a player's hand, but rather the player's discards. In addition, the hand may or may not be at tenpai upon scoring. By default, this hand is set to mangan.