A card that is legal is, in general, a card that is allowed for use in official formats in the TCG and/or OCG. The Card Legality online list can verify whether a released set or card(s) are legal or not.
Cards released in sets are legal upon the official release date of the booster packs, or in the case of Turbo Packs, on the start date for the prizing product. A set only becomes legal once it has been released in a specific country. Duelists should always verify that cards are legal for play in their specific region, and not rely on legality dates for other areas.
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 SHONEN JUMP Magazine, SHONEN JUMP Alpha subscription, and the Yu-Gi-Oh!-related graphic novels are legal only on continent(s) where they have been officially released. If a promo card is reprinted (such as in a booster set, Structure Deck, Tin, etc.) then it becomes legal for use, no matter how it was originally released.
Cards obtained as subscription promos for SHONEN JUMP Magazine or SHONEN JUMP Alpha are legal on the date specified for that card on the Card Legality online list.
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 Sanctioned 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 Sanctioned events in Japan and parts of Asia.
Cards that have been altered from their original appearance with surface decoration are legal for play only if the alterations do not obscure any portion 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. For example, if a player has a copy of "Spore" that has had slight alterations done to the card image with a marker, the player must ask the officials judge if the card is allowed, and abide by their decision.
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 they must be kept outside of the player's Deck.
Note that while cards on the Forbidden list are banned from Advanced Format tournaments and the OCG, which comprise the vast majority of official Duels, such cards are still legal, as they are allowed in the Traditional Format or in certain Special Formats or events, and 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) will never be allowed in any official format.