No? Well, you should now because it has recently been added to the Oxford Dictionary of English (not to be confused with the Oxford English Dictionary)!
This newly added noun, means a “a person who avoids social contact” with origins from Japan where there is an evident abnormal avoidance of social contact, typically by adolescent males, isolating and often confining themselves to their home. From ANN, this “concept has been depicted in anime several times, most notably in Welcome to the NHK, …Sayonara Zetsubou Sensei, Eden of The East, Rozen Maiden, and others. Japan itself is depicted as having been a hikikomori prior to meeting America in the manga Hetalia – Axis Powers.”
Otaku has previously been added in the 2005 edition of Oxford Dictionary of English. No news of whether anime or manga have been added to the dictionary already but I have a feeling it will be added in due time.