What is "penalty card"

A penalty card is used in many sports as a means of warning, reprimanding or penalising a player, coach or team official. Penalty cards are most commonly used by referees or umpires to indicate that a player has committed an offense. The referee will hold the card above his or her head while looking or pointing towards the player that has committed the offense. The colour and/or shape of the card used by the official indicates the type or seriousness of the offence and the level of punishment that is to be applied.

By analogy the term is sometimes used in non-sporting contexts. For example, the UK Radio Authority spoke of issuing a yellow card to those who broke its rules.