fair ball

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun baseball, softball A ball which has been hit between the first base and third base lines

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (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