Hentai (Japanese: ヘンタイ) is a style of Japanese pornographic anime and manga. In addition to anime and manga, hentai works exist in a variety of media, including artwork and video games (commonly known as eroge).

The development of hentai has been influenced by Japanese cultural and historical attitudes toward sexuality. Hentai works, which are often self-published, form a significant portion of the market for doujin works, including doujinshi. Numerous subgenres exist depicting a variety of sexual acts and relationships, as well as novel fetishes.

In sexual contexts, hentai carries additional meanings of "perversion" or "abnormality", especially when used as an adjective; in these uses, it is the shortened form of the phrase hentai seiyoku (変態性欲) which means "sexual perversion". The character hen is a catch-all for queerness as a peculiarity—it does not carry an explicit sexual reference. While the term has expanded in use to cover a range of publications including homosexual publications, it remains primarily a heterosexual term, as terms indicating homosexuality entered Japan as foreign words. Japanese pornographic works are often simply tagged as 18-kin (18禁; '18-prohibited'), meaning "prohibited to those not yet 18 years old", and seijin manga (成人漫画; "adult manga"). Less official terms also in use include ero anime (エロアニメ), ero manga (エロ漫画), and the English initialism AV (for "adult video"). Usage of the term hentai does not define a genre in Japan.

Hentai is defined differently in English. The Oxford Dictionary Online defines it as "a subgenre of the Japanese genres of manga and anime, characterized by overtly sexualized characters and sexually explicit images and plots." The origin of the word in English is unknown, but AnimeNation's John Oppliger points to the early 1990s, when a Dirty Pair erotic doujinshi (self-published work) titled H-Bomb was released, and when many websites sold access to images culled from Japanese erotic visual novels and games. The earliest English use of the term traces back to the rec.arts.anime boards, with a 1990 post concerning Happosai of Ranma ½ and the first discussion of the meaning in 1991. A 1995 glossary on the rec.arts.anime boards contained reference to the Japanese usage and the evolving definition of hentai as "pervert" or "perverted sex". The Anime Movie Guide, published in 1997, defines "ecchi" (エッチ, etchi) as the initial sound of hentai (i.e., the name of the letter H, as pronounced in Japanese); it included that ecchi was "milder than hentai". A year later it was defined as a genre in Good Vibrations Guide to Sex. At the beginning of 2000, "hentai" was listed as the 41st most-popular search term of the internet, while "anime" ranked 99th. The attribution has been applied retroactively to works such as Urotsukidōji, La Blue Girl, and Cool Devices. Urotsukidōji had previously been described with terms such as "Japornimation", and "erotic grotesque", prior to being identified as hentai.

変態 (hentai; shinjitai; (listen^ⓘ)) derives from 變態 (classical Chinese, also kyūjitai), which is attested in classical Chinese texts. It functioned as a verbal phrase, from its two component morphemes, 變 meaning "to change" and 態 meaning "state" or "condition," hence "to change from one state to another." Literal one-to-one English translations for this would be transform and metamorph, wherein trans- and meta- correspond to 變, while form and morph to 態. This meaning is attested in early Middle Japanese and later texts.

In Chinese, 變 is primarily a verb meaning to "change," and secondarily a noun meaning "troublesome event," but in Japanese, it was extended to an adjectival noun meaning "different," "unusual" or "strange" (compare other compounds such as hentai (變體; lit. 'unusual form') as in hentaigana (變體假名), and henkaku (變格; lit. 'unusual style') as in henkaku katsuyō (變格活用)). This led 變態 to become a noun phrase meaning "strange state," thus "abnormality," instead of the original "to change to another state," in modern Japanese and Chinese publications, particularly in psychology and physiology. A psychological application of this meaning is found in the phrase 変態性欲 (hentai seiyoku lit. 'abnormal sexual desire'), which has been cited as being shortened (by ellipsis) back to just 変態.

Yet another meaning, metamorphosis, which resembles the original one, was first adopted by the Entomological Society of Japan and reintroduced into Chinese, Korean and Vietnamese. This meaning is used in the English translation of the light novel Perfect Blue: Complete Metamorphosis, although it is not directly entomological, and it may also reference the semi-sexual portions of the work.

It is worth noting that 変態 is further shortened to H (etchi), the first letter of its romanization. Both hentai and etchi (or ecchi in English parlance) are used to refer to sexual perversion/deviance, or people therewith, as well as smut. A person accused of sexual perversion may be derogatorily called hentai, while a sex scene in a film, TV show or erotic game is called Hシーン (etchi shīn lit. 'H-scene'). The distinction outside of Japanese contexts between "hardcore" hentai and "softcore" etchi is entirely artificial.