: 100 Japanese pornographic works are often simply tagged as 18-kin ( 18禁, '18-prohibited'), meaning 'prohibited to those not yet 18 years old', and seijin manga ( 成人漫画, 'adult manga').

Less official terms also in use include ero anime ( エロアニメ), ero manga ( エロ漫画), and the English initialism AV (for 'adult video'). Usage of the term hentai does not define a genre in Japan. Hentai is defined differently in English.

The Oxford Dictionary Online defines it as 'a subgenre of the Japanese genres of manga and anime, characterized by overtly sexualized characters and sexually explicit images and plots.' The origin of the word in English is unknown, but AnimeNation's John Oppliger points to the early 1990s, when a Dirty Pair erotic doujinshi (self-published work) titled H-Bomb was released, and when many websites sold access to images culled from Japanese erotic visual novels and games. The earliest English use of the term traces back to the boards with a 1990 post concerning Happosai of Ranma ½ and the first discussion of the meaning in 1991.