A bid made after partner has shown one or more unspecified suits by means of a conventional bid that is to play if and only if the bid suit coincides with one of partner’s possible suits.

For example:

Facing the Gambling 3NT opening, which usually shows a solid minor and little outside, West decides he has too few stoppers to pass 3NT. He bids 4♣ expecting East to pass with a long suit of clubs and otherwise to correct to 4.