What is a catch?

[leonardfan40]

Had the ball, got his feet down. Then landed out of bounds and the defender kept trying to get the ball loose out of bounds. How can a defender be allowed to dislodge a ball when the WR is already out of bounds and then rule it a no catch? The catch rule has always been a joke. Nobody knows what a catch is and apparently a defender can make it a no catch well after the play is over now.