Defensive three-second violation

A defensive three-second violation, also known as illegal defense, is a basketball rules infraction in the National Basketball Association (NBA) introduced in the 2001–2002 season. It is assessed when a member of the defending team spends more than three seconds in the free throw lane (also known as the 16-foot lane, or colloquially as "in the paint") while not actively guarding an opponent. To be considered actively guarding, a defender must be within arm's length of an opponent and in a guarding position. A three-second count is suspended if:

• a player is in the act of shooting
• there is a loss of team control
• the defender is actively guarding an opponent
• the defender clears the painted area
• it is imminent the defender will become legal

In addition, a player guarding an opponent with the ball may be in the paint without actively guarding the opponent.

The team committing a defensive three-second violation is assessed a team technical foul. The offense will receive one free throw and retain possession of the ball.

Prior to the 2001–2002 NBA season, any form of zone defense was considered an illegal defense violation and resulted in a warning on the first violation and then a technical foul for any subsequent violations. However, the defensive three-second violation 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 Philippine Basketball Association used to follow the illegal defense rule until the 2003 PBA season when it was abolished and reverted its rule starting in the 2015–16 PBA season. The high school game does not use this rule, nor does European basketball.

The WNBA started using it in 2013.