Google Glossary (which is very cool) defines Hentai as:

• Literally meaning "pervert". Can be used to refer to someone, but is also used for a particular genre of anime. Hentai is not porn, although much of hentai could be called pornographic. As the translation implies, it's something far more bizzare. It ranges from rather light but explicit sex to demons raping schoolgirls with their tentacles. One important note here is that even when hentai is most like regular porn, it still has about a million times more plot. See also nudity.

http://www.geocities.com/cryptochrome/char/glossary.htm

• A Japanese slang word meaning perverted or perversion. In the world of manga and anime, hentai refers to the Adult oriented titles that present extreme, graphic sexual imagery. A common Western stereotype concerning manga and anime is that these artforms are filled with graphic sex. While such imagery exists (and has always existed in Japan), hentai titles make up just a small niche market in Japan's massive manga and anime output. Good examples of hentai anime would be La Blue Girl and Urotsukidoji. (also see SHUNGA).

http://www.theblackmoon.com/Gloss/agloss.html

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