Popular convention in the game of Bridge to determine if the side has enough controls to play in a slam.
Generally a
bid of 4NT asks partner to respond with an artificial bid dependent on the number of
aces he holds.
The most popular variation is Key-Card which counts the king of the implied
trump suit as a key-card. Other variations allow for more than one key suit in which they king counts as a key card.
1S -
2H - 4NT
4NT is Blackwood. According to which variation you are playing you are asking for
aces only or possibly the presence of the king or spades or
king of hearts as well as a keycard.