yellow card
noun [ C usually singular ] uk/ˌjel.əʊ ˈkɑːd/ us/ˌjel.oʊ ˈkɑːrd/
(足球比赛中的)黄牌
in football, and some other sports, a small, yellow card that is shown to a player by the referee (= the official who is responsible for making certain the rules are followed) as a warning that the player has not obeyed a rule