In 2001, the editors of the New Oxford American Dictionary purposefully inserted a fake word into their 2001 edition, in an attempt to fool other lexicographers (people who are involved in the making of dictionaries) into proving that they stole material from them. If another dictionary cited the word that the NOAD had invented, it showed that they were simply lifting words from them.
The word was esquivalience, and the fake definition given was (rather aptly) “to shirk your duties”.
Predictably, the word was picked up by dictionary.com (who later removed it when they found it it was a fake word), as well as Google Dictionary.
Given that English has no official governing body, since the word has now been featured in a dictionary, one could argue that it is now an official English word – though such things are usually decided by popular usage.