In basketball, an illegal screen is a penalty that occurs when the screener (the player who sets the screen) is in violation of the rules for setting screens in the sport. Screeners must be stationary when setting their screens, and are not allowed to lean into or make purposeful contact with the opponent they are screening.
When the screener does not remain stationary once the screen has been set, they can be called for an illegal screen. If the screener sticks an arm or leg out to slow the movement of the opponent being screened, this is also considered an illegal screen.
While illegal screens are a basic penalty to avoid, they can be very costly for the offense. If a referee calls an illegal screen it will result in a turnover for the offense and the opposing team gaining possession. Illegal screens are also commonly referred to as a "moving pick".