A Japanese word used to define an obsessive person. In English culture, it's used mainly to define persons that are obsessed with "anime", or Japanese animation.
Usage: "My girlfriend is such an otaku - she was up ALL NIGHT reading that 'Bleach' manga and she forgot to call!"
An obsessive compulsive fan. They obsess over a topic/genre to the point of unhealthiness.
Usage: The otaku lined up for 3 days without bathing to get the new game as soon as it was released.