The half-court line (also known as the 'midline') is a line that spans the entire width of the basketball floor, splitting the court into two equally sized sections. Once the ball crosses the midline, it cannot be carried back to that half of the court by an offensive player.
If the ball handler is caught stepping on or over the midline after the ball has already been advanced to one side of the court, the referee will stop play to call a 'backcourt' violation. This results in a turnover, with the offense conceding possession of the ball to the opposing team.
Backcourt violations are usually incurred when an offensive player is being careless and demonstrates a lapse of awareness. Players that fail to pay attention to their whereabouts on the court, especially when faced with pressure from opposing defenders, are extremely susceptible to stepping on the midline with the ball in hand.
Rare instances exist in which offensive players are permitted to obtain the ball behind the midline. If a defensive player hits the ball back over the line, members of the offense are permitted to cross the boundary and retrieve the ball without being penalized. In addition, a backcourt violation is not called if the referee deems that the ball handler was pushed over the midline by a defender, resulting in a foul being called on the defense.