From mlb rule book, seems to be up to the umpire's judgement to determine "secure possession" A CATCH is the act of a fielder in getting secure possession in his hand or glove of a ball in flight and firmly holding it; providing he does not use his cap, protector, pocket or any other part of his uniform in getting possession... In establishing the validity of the catch, the fielder shall hold the ball long enough to prove that he has complete control of the ball and that his release of the ball is voluntary and intentional.