In baseball, a fair ball is a batted ball that entitles the batter to attempt to reach first base.
By contrast, a foul ball is a batted ball that does not entitle the batter to attempt to reach first base.
Whether a batted ball is fair or foul is determined by the location of the ball at the appropriate reference point, as follows: If any part of the ball is on or above fair territory at the appropriate reference point, it is fair; otherwise, it is foul.
Fair territory or fair ground is defined as the area of the playing field between the two foul lines, and includes the foul lines themselves and the foul poles.
[1] However, certain exceptions exist: On a fair ball, the batter attempts to reach first base or any subsequent base, runners attempt to advance and fielders try to record outs.