Illegal

Illegal  cards are cards that are cannot be used in official tournaments and duels (including those unoffcial ones those are run on forums/social sites), unless it is a casual duel with a friend if both players agree on the terms of the game.

Illegal cards can be distinguished with the that replaces the card id in the left bottom corner of the card.

In other Video Games:
In YGOPRO percy, Illegal cards have the tag [Anime], despite the fact that they are real cards.