Wiktionary
free substitution
n. (context sports English) A rule in some sports that allows players to enter and leave the game for other players many times during the course of the game; and for coaches to bring in and take out players an unlimited number of times.
Wikipedia
Free substitution
Free substitution is a rule in some sports that allows players to enter and leave the game for other players many times during the course of the game; and for coaches to bring in and take out players an unlimited number of times.