Meaning 1 ►▼
a foul ball is a ball hit by the batter outside the baseball line of play
The first ball hit by the batter was a foul ball and declared a strike by the umpire. Fouls count as strikes except for the third foul.
Meaning 2 ►▼
a foul ball is generally ineffective; it is out of the field of play and is not relevant to the mission.
He hit a foul ball. His comment was not effective; it did not accomplish anything!