fair ball

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Baseball A batted ball that first strikes the ground or leaves the playing field beyond first or third base within the foul lines or that is within the foul lines as it bounces past first or third base or that comes to rest or is touched by a fielder in front of first or third base within the foul lines.

Wiktionary

1. n. baseball, softball A ball which has been hit between the first base and third base lines

GNU Webster's 1913

1. adj. (Baseball) A batted ball that falls inside the foul lines; -- called also a fair hit.

WordNet 3.0

1. n. (baseball) a ball struck with the bat so that it stays between the lines (the foul lines) that define the width of the playing field