Agile Opportunist is a paragon tier feat available to 11th level or higher characters.
When a character with the Agile Opportunist feat is subjected to forced movement—push, pull, or slide—into a square adjacent to an enemy, the character can make a melee basic attack against that enemy as an immediate reaction to the forced movement.[PH2:190][U :11/2009]
The November 2009 update changed the melee basic attack granted by the Agile Opportunist feat from an opportunity action to an immediate action, to limit the number of attacks possible per round.