yellow card

Short definition from Wiktionary

Noun

1. 1
   a yellow-coloured card, shown to a player (especially in association football) as a caution that he or she has committed a flagrant foul; a further such foul would result in a red card.
2. 2
   one of various yellow-colored cards used to verify the healthiness, vaccination status or other medical info of travelers.
3. 3
   a form to record safety concerns and adverse reactions to medicinal drugs.
4. 4
   A card sent out by a travelling dance or theatre company to request temporary personnel from a local labor union.
5. 5
   A yellow ticket.
6. 6
   a card given to British Army personnel during The Troubles.

Verb

1. 1
   To show someone a yellow card