From Bulbapedia, the community-driven Pokémon encyclopedia.
The Unlimited format is a format of the Pokémon Trading Card Game in which any card that was once playable in tournaments (that is, all cards except Worlds edition decks, jumbo cards, Ancient Mew, Birthday Pikachu, and otherwise unplayable cards) can be used. It is not as popular or as commonly used as the Modified format and is not used in any tournaments sanctioned by Pokémon Organized Play. It can, however, be used in Pokémon Leagues if the League Leader wishes to allow it.
Because of the lack of restrictions in Unlimited, decks can use combinations of cards which are not legal in Modified play. Cards such as Porygon2 and Seeker which never appeared in the same Modified format, are frequently used together. Despite the large variety of cards available, only a few strategies make up what is generally considered the highest "tier" in Unlimited play, with most competitive decks being either "donk" decks focused on winning by removing all of the opponent's Pokémon from play, or "lock" decks focused on preventing the opponent from using attacks, abilities, and/or Trainer cards.