DICTIONARIES ARE DESCRIPTIVE, NOT PRESCRIPTIVE. THEY DOCUMENT HOW WORDS ARE COMMONLY USED, THEY DO NOT DEFINE THEIR MEANINGS.
http://harpers.org/wp-content/uploads/HarpersMagazine-2001-0...
(I only comment because I'm amused to find I've heard "cromulent" used as a "real word" enough that my brain has intuited rules for its usage.)