A card that, whilst not yet a winner, may become a winner if the opponents weaken their holding in the suit by discarding from it.
The main use of the term is in connection with squeezes. For example, if North holds ♥A-K-Q-J-4 and East ♥10-8-6-5-3, the ♥4 is a threat card (menace). East cannot afford to discard a heart for fear of establishing the ♥4 as a winner.