Definition
A safe tile is a discard that cannot complete any hand suggested by the exposures on the table, usually because all its copies are already visible or no exposed set can use it.
The safest discard is a tile of which three copies are already out, since nobody can be holding a pair of it. Defensive play late in a hand is mostly the art of finding safe tiles without wrecking your own hand.
Read the full guide: What Not to Discard: Defensive Play in American Mahjong.
See it on real tiles: the coach reads any hand and names its direction, free, at the free read. Members get the coach on call for their own games.
More terms: Pung, Kong, Quint, Pair, Charleston, Blind pass, Courtesy pass, Joker.