A card that can play as a wild card, but only in certain situations.
Example: In certain poker variants, a joker can only complete a flush or straight, but cannot be used to make a pair, trips, quads, or 5 of a kind. The joker is playing as a bug.
In certain variants of lowball, the joker plays as the lowest nonpaired card. This kind of wild card is also referred to as a bug.