Kong

A kong is a meld of all four copies of one tile. There are three ways to form a kong: concealed (declare all four from your hand), melded (call the fourth tile from a discard), or upgraded (add the fourth tile to an existing open pung). After declaring a kong you draw a replacement tile from the dead wall — which can itself complete a winning hand (Rinshan Kaihou in Riichi, Kong on Kong in HK).