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They use this terminology to describe a foul where neither team is in immediate control of the ball. This is important in the NBA, since they don't count offensive fouls in the team foul count (for shooting penalty). In FIBA, if player A1 fumbled the ball and while the ball was rolling around loose on the floor, player A2 fouled opponent B1 while trying to recover the ball, it would be an offensive foul, since Team A is still deemed to have team control until a shot is taken or Team B gains possession. In the NBA, that would be a loose ball foul. A rebounding situation is the most common type of "loose ball foul."
I googled using key phrase "NBA loose ball foul". It came up on the first page. :)