a fly1 (sense 25) or , line drive that is caught or touched on the fly in fair territory or lands on or inside either foul line in the outfield, or a grounder that is touched in fair territory, comes to a stop in the infield, or hits first or third base or passes either of them in fair territory: a fly that hits either of the foul poles or that sails between them as it leaves the playing field is also a fair ball