Outside of Japan, it seems to mean someone who's a fan of Japanese anime/manga. Within Japan, otaku can mean a person overly obsessed with anything. In Japan, the term is a negative one since it can be considered those with unhealthy obsessions...seemingly. Bottom line, otaku = Japan's word for nerd/geek, but they have an obsession with a certain thing. For example, in Detective Conan, Shinichi is referred to as a Holmes Otaku, because he's a super obsessed fan of Sherlock Holmes mysteries.