(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 WordNet 3.0 & Open English WordNet

In baseball, a foul ball is a batted ball that: Settles on foul territory between home and first base or between home and third base, or Bounces and then goes past first or third base on or over foul territory, or Has its first bounce occur in foul territory beyond first or third base, or Touches the person of an umpire or player, or any object foreign to the natural ground, while on or over foul territory. Wikipedia

In baseball, generally, a struck ball that ends up in foul territory Wikidata

A batted ball that lands in foul territory. Wiktionary

Ball that land in foul territory. Wiktionary (translation)