A defensive strategy in which one (or more) players leaves his man or his zone to protect the key or an area near the basket. All zone defenses essentially are required to collapse on the ball when it enters the key.
The best defensive players are experts at leaving their man or their zone in order to block a shot or impede a ball handler's drive to the basket. At the same time, the best passers can usually exploit a collapsing defense by finding a vacated teammate for an open shot.
A defensive player may also collapse to the lane to draw a charge from an attacking ball handler. In order to do that in the NBA, the defensive player has to be outside of the semi-arc four feet away from the basket.