界内球为棒球术语，是打击者将球击出后，落点在左右界线延伸至全垒打墙以内的范围，或是全垒打时通过标竿内侧或正好击中标竿。

In baseball, a fair ball is a batted ball that entitles the batter to attempt to reach first base. In order for a batted ball to be fair, it must be hit in such a way that it:

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.