Real Dictionary

ball boy
countable noun: In a tennis match, the ball boys pick up any balls that go into the net or off the court and throw them back to the players. In a baseball game, the ball boys are in charge of collecting the balls that are hit out of the field.

ball boy in American English
a boy who retrieves balls that have gone out of play, as in tennis and baseball, or a boy in charge of the extra balls used in practice, as in basketball and football
noun: an attendant, usually a boy or young man, employed to retrieve balls and supply players with them

ball boy in British English
noun: (in sport, esp tennis) a person who retrieves balls that go out of play