"Eveywhere" in hoopy_frood's writeup should be understood in the American sense of the word, since Scrabble is available in many different language versions. Even among English speakers, the standard dictionary used for official tournament Scrabble in the UK and Ireland is the Chambers dictionary, which has traditionally been favoured for word games over the various editions of the OED (generally, if unfoundedly, considered the most authoritative British English dictionary - professional writers and their ilk tend to go for the single-volume Longman or Collins English dictionaries, but I digress) because the latter list all their headwords with initial caps, making it difficult to distinguish proper nouns. Chambers also publish Official Scrabble Words, a definition-free wordlist drawn from the dictionary including all valid inflected forms of words from 2 to 9 letters, and Official Scrabble Lists, which lists words on the basis of various game-related criteria.
For transatlantic tournaments (including the anglophone version of the World Championships), players are allowed to use words from both British and American lists. In fact, both lists include both British and American spelling variants, but there are differences with the obscure two- and three-letter words that are so vital for tournament play.
The equivalent work for French-speaking Scrabble players, on both sides of the pond, is the Officiel du Scrabble published by Larousse under the aegis of the Fédération internationale du Scrabble francophone, which does include definitions.