Otaku is the Japanese direct equivalent of "nerd" in English.
In English otaku is used to describe a nerd nerding over Asian stuff (anime, manga, games, whatever). Some gatekeep it for Japan-only stuff, while others use it for K-pop and K-drama as well.
In Japanese it's the opposite - they use "nerd" to decribe people nerding over American stuff.
Edit: both loaned versions are a bit more derogatory than their "native" version.