Defensive Sweep

 
Prerequisite(s): Base Attack Bonus 15+
 
Benefit: If an opponent begins his turn in a square that you threaten and does not move, he provokes an attack of opportunity from you immediately after his turn ends. Any sort of movement, including a 5' step, allows the opponent to avoid the special attack of opportunity. This feat does not give you the ability to make additional attacks of opportunity in a given round.
 
Source: Players Handbook II