As has been mentioned, there's no defining body, so to speak. The Oxford English Dictionary is the English nerds' standard, though of course, it's not foolproof by any stretch--for a start, citations can by definition only be the first found recorded usage of a word.
When you have a majority of English nerds on your side, you probably have correct usage. Probably. And, yes, we know that there's archaic terms a-plenty that are no longer considered correct. Frankly, we know more about it than non-English nerds. ("A norange," anyone?) However, we do recognize that consensus is, on this, important to comprehension.
We do all know that English is a changing language--all languages are, no matter how much the French may be officially fighting it. However, there is a reason we try to hold the positions we do for as long as we can. If we didn't, how would we understand each other? How would you teach English as a second language if it wasn't clear as a first one?
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"