Fourth-suit forcing is the popular term for the idea that a bid by responder of the only unbid suit at his second turn is an artificial force.
Most play that it is forcing to game, although some play it is forcing one round only. The bid is made when responder has doubts about strain or level.
Suppose you hold:
K 2 
A K J 4 2 
Q 10 3 
8 5 3
You are South and this is the bidding:
North South
1 1
1 ?
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