If you try to google "hentai definition" most result would define hentai as a Japanese genres of manga and anime with sexually explicit images and plots. But Wikipedia explains it otherwise:
In Japanese, the term describes any type of perverse or bizarre sexual desire or act; it does not represent a genre of work.
Internationally, hentai is a catch-all term to describe a genre of anime and manga pornography.
And from this question we know that the word hentai never means anime porn in Japan.
No, hentai is a typical "英製和語" that has gained a totally different meaning outside of Japan. It never means anime porn in Japan.
From that question, OP mentioned that it was originated from 4chan, but from some comment on that question, the word it self was already in use for Japanese animation porn long before 4chan.
So how come non-Japanese refer to anime and manga pornography as hentai? And when did it start?