A legal Card is a Card that is allowed for use in official tournaments.
Released in Sets
Cards released in sets are legal upon the official release date of the booster pack or in the case of Turbo Packs, on the start date for this prize product. A set only becomes legal once it has been released in the country you want to play it in.
Preview Cards from an upcoming set that are distributed in tins before the set is released become legal on the official release date of the tin.
Cards obtained from the United States Duel Terminal sets are only legal if they have been released in another product.
Video game promo Cards are legal starting with the official “on sale” date for that video game within that Continent.
Promo Cards from the SHONEN JUMP Magazine are legal only on continent(s) where they have been officially released. If a promo Card is reprinted (such as in a booster pack, Structure Deck, Tin, etc.) then it becomes legal for use, no matter how it was originally released.
TCG and OCG
Yu-Gi-Oh! Championship Series (YCS) prize Cards become legal on a continent once a YCS or WCQ awarding that Card has been held on that continent.
TCG Cards printed in English, French, German, Italian, Portuguese, or Spanish are legal for play in official tournaments within all TCG territories. Cards printed for the Asian version of the game, known as the OCG, are usually printed in Japanese or Korean, and are legal for play only in official events in Japan and parts of Asia.
Cards that have been altered from their original appearance with surface decoration are only legal for play if the alterations do not conceal any part of the Card text, make the illustration difficult to recognize, affect the Card’s thickness or texture, or make the Card distinguishable from other Cards in the Deck while it is face-down.
On extremely rare occasions, miscut or misprinted Cards are legal if they are not distinguishable from other Cards in the Deck while face-down. The player must provide an accurate, local-language copy of the correct Card text or a correct local-language copy of the Card to show to opponents or tournament officials. If Cards are used to supply the proper Card text, then you must keep them outside of your Deck.
Note that Cards on the Forbidden list that are banned from Advanced Format tournaments are still legal, so long as they are allowed in the Traditional Format or in certain Special Formats or events. They may later come off the Forbidden list as it changes every several months. In contrast, Cards that are illegal (such as the original Egyptian Gods; “Obelisk the Tormentor”, “Slifer the Sky Dragon”, “The Winged Dragon of Ra”) will never be allowed in any official format. This is mentioned on the bottom of the Card.