Real Dictionary

ball girl
a girl who retrieves balls that have gone out of play, as in tennis and baseball, or a girl in charge of the extra balls used in practice, as in basketball and football
countable noun: In a tennis match, the ball girls 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 girls are in charge of collecting the balls that are hit out of the field.
noun: an attendant, usually a girl or young woman, employed to retrieve balls and supply players with them