A person obsessed with anime, manga, comics, entertainment, video games, electronics, toys, vehicles, weapons, history or other subjects.
The term started as a derogatory term in Japan during the late 70s towards anti-social people who are highly fixated over a particular subject or person, but has since evolved (in the west mostly) to imply a cool, eccentric individual with particularly unique hobbies or interests. However, depictions of generic otaku as an "unhealthily obsessive fan" stock character remains in widespread uses in media (specially in Japan).
Tag what you see: The tag is only applicable if the image shows their otakuness.