Illegal cards are cards that are cannot be used in official tournaments and duels (including those unofficial 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 thethat replaces the card id in the left bottom corner of the card.
Unlike other cards, Illegal cards never fall on the ban list.
Most of Illegal cards are either(Except " " which is Forbidden instead), Original Versions of cards, counterfeit cards...)
In other Video Games:Edit
In YGOPRO Percy, Illegal cards have the tag [Anime], despite the fact that they are real cards, and not appearing in any anime.