Blackwood
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.