From MLB.com Official Rules, rule 2.0 A FOUL BALL is a batted ball that settles on foul territory between home and first base, or between home and third base, or that bounds past first or third base on or over foul territory, or that first fall... read more ...
(baseball) a ball struck with the bat so that it does not stay between the lines (the foul lines) that define the width of the playing field
a strike,'' Hernandez said through a translator
A ball that lands outside the first or third base foul lines.
A ball hit outside the foul lines.
A batted ball that lands on foul territory between home plate and first base or third base, bounds past first or third base on or over third territory, first touches foul territory beyond first or third base, or touches a player, umpire or any object not part of the playing field while over foul territory.
A ball that hits the ground to the outside (the right or left) of the foul lines before passing the 1st or 3rd base bag. Remember that batted balls often curve towards the foul lines.
A ball that is hit that does one of two things. In the infield, the ball comes to a stop outside of the baseline (it may roll foul and come back in fair, and still be ok). In the outfield, however, if the ball lands outside the baseline, it is foul. It counts as a strike against the batter every time.