Definition of fair ball in English:

fair ball

noun

  • 1A pitch delivered within the strike zone (now rare).

  • 2A batted ball that keeps within the two foul lines as it goes past first or third base, or that comes to rest or is touched by a fielder between the foul lines in the infield.

Origin

Mid 19th century; earliest use found in The Spirit of the Times: a chronicle of the turf, agriculture, field sports, literature and the stage.