Stayman
(see also: Puppet Stayman and Smolen)
Over 1NT or 2NT opening, the cheapest minor bid is used to ask partner for a four card major. Partner shows their four card major (preferring to bid hearts if opener has both), or bids the cheapest denomination of diamonds to deny a four card major.
Example:
Partner Opponent You
1NT Pass 2 C (Stayman)
2 D = denies a four card major 2 NT (You must bid this after a denial)
or 2 H = I have 4 hearts - might have 4 spades too
or 2 S = I have 4 spades + I don't have 4 hearts.
Your responses can be:
2 NT - If partner bids 2 D
Pass - If partner bids your 4 card suit, you have a weak hand and you're not
playing Forcing.
Raise - If partner bids your 4 card suit and you have a good hand (10+)
Non-forcing Stayman: This means that after opener makes their response, a new suit bid by responder is not forcing.
Forcing Stayman: This means that after opener makes their response, a new suit bid by responder is forcing for one round.