surplus tile; unnecessary tile at tenpai
In mahjong, a tile that is no longer needed to complete the hand, especially the tile discarded when reaching tenpai (ready hand).
See also: テンパイ
聴牌したら、余剰牌を捨てて待ちを宣言する。
When you reach tenpai, you discard the surplus tile and declare your wait.
どの牌が余剰牌かを見極めるのが難
It's difficult to determine which tile is the surplus one.
Compound of 余剰 (surplus) and 牌 (tile), a straightforward mahjong term.