noun
waiting for a single tile to complete a pair and win; waiting for one tile to finish a pair with four melds already completed
Mahjong term: a tenpai pattern where the hand needs only one specific tile to form the final pair (head). The first gloss describes the general case; the second describes a specific situation where all four melds are already complete and only the pair is missing.
単騎待ちでリーチをかけた。
I declared riichi with a single-tile wait for the pair.
彼は単騎待ちのまま、最後までその牌を引けなかった。
He stayed on a single-tile wait for the pair and never drew that tile until the end.
待ち is a general mahjong term for 'wait' (the tiles needed to win); 単騎待ち specifies a wait for exactly one tile to complete the pair.
Compound of 単騎 (single horseman, here meaning 'single' or 'alone') and 待ち (waiting). The term is specific to mahjong.