This Words With Friends Mobile Cheat (dictionary) is based on the Enhanced North American Benchmark Lexicon, also known as "ENABLE". This dictionary is a public domain word list with over 170,000 entries and is used by many word games. It is not the same as the Scrabble dictionary, which is why words like "ZA" are legal in Words With Friends, but not Scrabble.
While based on the ENABLE word list, words are occasionally added to the dictionary to meet player demands and varies slightly from version-to-version. Since the dictionary is built into the app, it creates the following odd situation. If opposing players are using different revisions of the game, they may be playing with different dictionaries. So in affect, Words With Friends has no official dictionary.
Unlike Scrabble, there are no word challenges in Words With Friends because the game will not allow an invalid word to be played. Players may try words until they are accepted. This technique is known as plugging.