A violation called when a defensive player spends three seconds or more inside the key without actively guarding a player. A defensive player is considered actively guarding someone when he's either guarding someone with the ball or within arm's length of an offensive player. The team committing a defensive three-second violation is charged with a technical foul, and the offense is awarded one free throw and possession. This is also known as "illegal defense".
The defensive three-second violation is what makes it difficult for NBA defenses to play zone, since zone defenses usually position a player in the middle of the key to stop penetration. The defensive three-second violation is not called in either high school or NCAA basketball.