Official Tournament and Club Word List
Official Tournament and Club Word List or Tournament Word List, referred to as OTaCWL, OWL, or TWL, is the official word authority for tournament Scrabble in the USA, Canada and Thailand. It is based on the Official Scrabble Players Dictionary (OSPD) with modifications to make it more suitable for tournament play. Its British counterpart is Collins Scrabble Words, and the combination of the two word lists is known as SOWPODS.
North American tournament Scrabble currently uses the second edition of OWL (variously referred to as OWL2, OTCWL2, TWL2, and TWL06). This version was created by the National Scrabble Association (NSA) Dictionary Committee in late 2005 and took effect on March 1, 2006. When authority for competitive Scrabble play in the United States and Canada was transferred from the NSA to the North American Scrabble Players Association (NASPA) on July 1, 2009, the NASPA Dictionary Committee reaffirmed the existing lexicon before beginning work on the next edition. 
OWL2 contains every word in the fourth edition of the Official Scrabble Players Dictionary, but also includes words considered unsuitable for that book.
Early printings contained several errors, which have been corrected on the NASPA web site.
The decision to bowdlerize the OSPD's third edition by removing a large number of possibly offensive words necessitated a separate, unabridged word list for tournament use. The first edition of OWL was created by the NSA Dictionary Committee, chaired by John Chew, and took effect on March 2, 1998. To avoid controversy, it was available for sale only to NSA members, and unlike the OSPD, did not include definitions. To provide additional value for tournament players, the OWL includes words whose base or inflected forms have up to nine letters, rather than the OSPD's eight. OSPD3 was created from OSPD2 by adding all appropriate words that had been added to Merriam-Webster's Collegiate Dictionary (MWCD) in the interim; OWL was then formed by restoring the possibly offensive words removed from OSPD3, and adding nine-letter words from MWCD.
Unlike the Official Scrabble Players Dictionary, OWL is a list and does not include definitions. OWL also contains words not included in OSPD because they are considered offensive, and a number of other additional words (mostly registered trademarks). Print versions of OWL can be procured from Merriam-Webster's customer service department by NASPA members only.
OWL is supplemented by the NSA Dictionary Committee's Official Long List, which includes all acceptable 10- to 15-letter words not already included in the OWL. The word list was compiled using Merriam-Webster's Collegiate Dictionary, 10th edition.