The division line, otherwise known as the midline or half court line, is the boundary line that splits a basketball court in half. Directly in the middle of the division line is the center circle (12 feet wide), where a player from each team competes for a jump ball to decide which team gets possession to start the game.
The main purpose of the division/half court line is to separate the court into two equal halves. The two sides are mirror images of one another, possessing the same dimensions and court markings. Each half extends 52 feet from the baseline (boundary line positioned a few feet behind the basket) to the division line and 25 feet from sideline to sideline. The division line represents a boundary that the team on offense must cross and stay in front of in order to score a basket. This rule effectively limits the area in which the offense can operate to just half of the court, forcing the offense to run a play and gain separation from the defense in order to have a realistic chance of scoring.
Once the team on offense crosses the division line, no player from that team is permitted to step on or over the line until a change of possession has occurred (the other team obtains the ball via a made/missed shot or a steal). If an offensive player travels behind the line for any reason, the referee will call a 'backcourt violation,' which means that the offense has officially entered the wrong half of the court and must forfeit possession of the ball to the opposition. The only time an offensive player is permitted to cross the boundary is to track down a ball that has been hit back over the division line by a member of the defense.
In the NBA, the team on offense has 8 seconds to advance the ball past the half court line upon inbounding the ball to begin play. If the ball handler does not manage to get over the line by the time the referee reaches a count of 8 seconds, the offense automatically loses the ball and the other team takes possession. At the collegiate and high school levels, teams have 10 seconds to cross the half court line before a violation occurs.