Anime Know-How: What Is Hentai?
If you are a fan of anime, then there is a really good chance you’ve stumbled upon the term [...]
Defintion
In Japanese, the term hentai loosely translates to 'perversion' or 'abnormality' in English. The word has a distinctly negative connotation abroad and can be used to insult some for their sexual preferences. Hentai is also a shortened term as it comes from 'Hentai Seiyoku,' a phrase which means 'Abnormal Sexual Desire.'

History
Hentai has a lot of baggage behind it. The word first appeared during the Meiji era and was used as a way to describe abnormal sexual fetishes. The word was used almost exclusively in the realm of science and scholarship, but it became popularized after World War II ended. The world hentai was then used as a slang term and shorted to either 'H' or 'ecchi' - but that slang now carries its own meaning these days. In Japan, the two words are often associated with the idea of lewdness and don't refer to explicit sexual content.
As the slang became popularized, the word hentai went on to gain a more delicate definition. While the term still referred to abnormal sexual fetishes abroad, the U.S. adopted the term as manga became a more global medium. Thanks to loose translations, western fans came to associate the word hentai with porn.

How It Is Used In Japan
In Japan, using the word hentai will force people to give you strange looks. It is not widely used abroad as the preferred term for such content is ero-manga or ecchi manga. If you do use the phrase, then you are referring to a very specific subset of animated pornography. Given hentai's clinical definition, any anime or manga described as such features an abnormal kink or fetish. For instance, titles featuring tentacle erotica or incest would be classified at hentai. So, make sure you speak accordingly.

How It Is Used In The West
For fans in the U.S., the word hentai is used interchangeably with porn. There is little distinction between the phrases due to loose translation efforts over the decade. As such, hentai has been divided into two subcategories in the west by fans. If you look into hentai, the overall genre will be divided into either softcore or hardcore. The phrase also covers the entire spectrum of pornography regardless of content or kink.
