Cartoon porn featuring girls unrealisticly hot that none of us will ever have.

A Japanese word denoting unfavorable (or favorable, depending on context and circumstances) sexual connotations.
A loose English translation would correspond to words like "perverse."
Contrary to popular belief (and to its usage in America), when used as a noun, this word denotes a person who fits this description and nothing more.

mostly what everyone else said. all i want is for someone to post an image. ^__^

the best thing in the world

The honed science of painstakingly designing young Japanese girls with the greatest rapability ever imagined.

Hentai (hen-tie) is Japanese for "pervert" or "perversion". In America it is often used to refer to Japanese animated pornography.

Saying the first letter in hentai (H) with a Japanese accent sounds like ecchi (eh'ch-chee). This is a slang word used in Japan that carries the same meaning as "hentai", although in America it is often used to denote soft-core anime porn.

Hentai is art