A holding in a suit, such as the ace, king, or maybe the queen, that is likely to be important on the given hand.
If you and your partner agree a minor suit below the level of 3NT, it is usual to play that bidding a new suit shows a feature (stopper) rather than length in the suit; eg 1♦-3♦-3♠ shows strength in spades (and by inference weakness in hearts).