Longman Dictionary of Contemporary English
n. 1 (context football rugby basketball English) A set-piece where the ball is thrown back into play 2 (context Australian rules football English) A throw of the ball back into play by the boundary umpire. The umpire faces away from the players and throws it in over his or her shoulder (so as to clearly not favour either side by where exactly it's thrown). 3 (context Ireland English) the start time of a Gaelic football match, comparable to kick off in other games.
n. (rugby) an act or instance of throwing a ball in to put it into play
A throw-in is a method of restarting play in a game of association football (or soccer) when the ball has exited the side of the field of play.