Definition
A joker is a wild tile that can stand in for any tile inside a pung, kong, or quint, but never in a pair, never as a single tile, and never in a hand marked jokerless.
An American set has eight. A joker in an exposed set can be redeemed by any player who offers the natural tile it represents.
Read the full guide: Joker Rules in American Mahjong: What Jokers Can and Can't Do.
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 exchange.