In Japan: an otaku is someone without a social life whatsoever and is an insult of the highest regard. There is a negative stigma associated to the phrase otherwise that involves a Japanese murderer who killed women because "they didn't have smooth faces like women in anime."
In America: a badge of honor worn by fans of anime, in lieu of simply calling themselves anime fans. Overused slang that has lost its meaning to many fanboys and fangirls.
In Japan: "With the amount of pornography that man buys, it's no wonder they consider him a disgusting otaku."
In America: "Dude, I love Sailor Moon and Utena because they have lesbians in it. I'm such an otaku! ^___^"
Otaku is rooted from the Japanese word オタク and can be used in many different ways the most common is refers to someone who is obsessed with Japanese culture specifically in Manga, Anime, Cosplay and such, The Japanese definition of Otaku is a nerd or weirdo.
"Gosh, Miranda Sings is such an otaku she is always watching some form of Anime."
"Look at him he is such an Otaku always playing with Pokemon trading cards."
an Otaku is a low life that is obsessed with anime. Not a weaboo though, so do not mix them up. Otaku's (yes there can be a group of otaku's. i prefer to call them a "bobs") Otaku's on the male spectrim are normally into hentai and large anime tits. Otaku's on the female spectrim are normally onbsessed with yaoi and like to see two men fuck. or they are just obsessed with male anime guys with abbs
normie: lol u weaboo Otaku: no in fact i am actually a otaku
normie: what the fuck