In professional sports, a replacement player is an athlete who is not a member of the league's players association and plays during a labor dispute such as a strike or lockout, serving as a strikebreaker.
Replacements related to strikes are mostly a North American phenomenon, since players in many other countries do not have league-wide collective salary bargaining (instead bargaining only with the club they're contracted to, and most commonly on an individual basis). A strike did however happen in the 2011–12 La Liga season in Spain, but no replacement teams were set up.
Elsewhere, replacement teams can occasionally happen through, among other things, illness breakouts (especially COVID-19) and aviation accidents.