There is no official academy of English as there is with French, Spanish and other European languages. The Oxford English Dictionary (OED) is the closest we have to such an authority.
How does the OED decide which words to include?
It accepts a new word if it is commonly used over a period of time. More on how this works here
Oxford Advanced Learner's Dictionary
Cambridge Advanced Learner's Dictionary (Dictionary & CD ROM)
Concise Oxford English Dictionary: 11th Edition Revised 2008
Oxford English Dictionary: 20 vol. print set & CD ROM