In trading card games, a promo card (promotional card) is a card from game makers that is not part of the "canonical" card series, and is given away with some other products that are somehow related to the thing that the card represents.

For example, Magic: the Gathering card Arena comes with the novel of same name, and Pokémon TCG promo cards are given away extremely liberally with Pokejunk (I have four Pokémon promo cards - call me a helpless victim of marketing =)