A person obsessed with any subject, whether anime, manga, comics, entertainment, video games, electronics, toys, trains, vehicles, weapons, history, etc.
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). In the West, otaku are often conflated with Western nerds, to the point of the term being used as a synonym, but each have their own specific differences socially and culturally. For the purposes of Danbooru though, they are one and the same.
Tag what you see: The tag is only applicable if the image shows their otakuness/nerdiness.