JAPAN:In Japanese was "Otaku" very polite form of "you" (literally your house). During the time the meaning has changed to hardcore fan of something usually manga and anime. Otaku has became used in meaning : nerd or jerk. Nowadays in Japan is the meaning returning back to non-offensive usage because of econimic trends.
ELSEWHERE: Out of Japan is the term "Otaku" used in the meaning of hardcore fan of anime and Manga. Many of anime fans call themselves otaku despite knowing it's meaning in Japan.