There’s a good chance you don’t know how to use the word Hentai in Japan! (NSFW Warning!)
Hentai (Hiragana: へんたい; Kanji: 変態) is an adult topic that has a drastically different meaning in Japan compared to English speaking countries.
In most English speaking countries, people use the word Hentai to refer to pornographic anime, manga, and even video games. If you typed the word Hentai into a search engine, there’s a good chance you’ll find adult-themed stuff like below (Below are images from the video game Bible Black)!
In Japan, Hentai is not used for pornographic media. Instead, the term Erotic (shortened to Ero エロ) is used in front of anime, manga, or video games (Erotic video games are shorted even further to Eroge エロゲ) when referring to pornographic or adult-themed media in Japan. The term Hentai actually refers to perverts or those who do perverse actions. A common example is someone who steals panties or wears panties on their head.
http://www.gamesradar.com/japanese-pc-magazine-gives-away-free-pair-of-panties-with-every-issue-enjoys-strong-increase-in-sales
I hoped I answered the question “What is Hentai?” And I hope you’re not scarred by what you found. You can find more information on subculture terms just like this in my growing Otaku Encyclopedia.