Restricted area arc
In the NBA, Euroleague, and starting in 2010, in FIBA and NCAA play, the key has an additional area, measured as an arc three feet from the basket[10] (collegiate), four feet from the basket (NBA), or 1.25 meters (approximately 4.1 feet) (FIBA). The area is officially known as the "restricted area" (RA) in the NBA, the "restricted area arc" in the NCAA and the "no-charge semicircles" in FIBA.[11]
Its purpose is to stop secondary defenders from taking a position under the basket in an attempt to draw the offensive foul when a player is driving to the basket. If an offensive player drives past his primary defender on the way to the basket and a secondary defender comes over, he must establish a legal position outside the RA to draw an offensive foul. If the drive starts inside the Lower Defensive Box (LDB – this is the area from the bottom tip of the free throw circle to the end line between the two 3’ posted-up marks), the secondary defender is legally allowed to be positioned inside the RA. The restricted area also does not apply if the secondary defender jumps in attempting to block the shot, the offensive player leads with his leg or knee in an unnatural motion or uses his off arm to prevent the defender from blocking his shot. The RA does not extend from below the backboard to the baseline. Therefore, if a player drives the baseline and is not attempting to go directly to the rim, the RA does not apply.
The restricted area arc rule was implemented in NCAA men's basketball for the 2010–2011 season. The NCAA approved adding a visible restricted-area arc three feet from the center of the basket in Division I men’s and women’s games for 2011–2012 season. The panel delayed implementation of the arc until the 2012-13 season for Divisions II and III to allow those schools time to plan and place the restricted-area arc in their home arenas.[12] Starting with the 2015-2016 season, the NCAA raised the RA arc to four feet from the center of the basket.
https://en.wikipedia.org/wiki/Key_(basketball)