Tanki, the lone or single-tile wait, is a tenpai hand made of four completed melds plus one isolated tile hunting for its twin to form the pair (atama). At most three copies remain unseen, which places tanki among the narrowest basic waits.
You win when you draw (tsumo) or claim (ron) the matching tile. In an ordinary hand, tanki is worth 2 fu. Suuankou tanki uses this pair wait. Kokushi musou may also wait on one specific terminal or honor, but it is a distinct special shape, not an ordinary tanki for fu calculation.
Tanki offers at most three unseen copies. It can sometimes keep a dora as the pair or target a tile judged easier to obtain, but its strategic value is contextual. Nobetan extends the idea across two tile types.