A system of responses to a 1NT opener. 2♣ shows a weak take-out type of hand with length in any suit. It demands 2♦, which responder can pass or convert to 2♥ or 2♠. 2♦ over 1NT is the Stayman enquiry (2NT denying a four-card major), while 2♥ and 2♠ are forcing with a five-card suit.
A jump to three of a major is a slam invitation.