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. EtymologyĪ depiction of a male homosexual couple from the January 1928 edition of Hentai shiryō.

The history of the word hentai has its origins in science and psychology. By the middle of the Meiji era, the term appeared in publications to describe unusual or abnormal traits, including paranormal abilities and psychological disorders.

A translation of German sexologist Richard von Krafft-Ebing's text Psychopathia Sexualis originated the concept of hentai seiyoku, as a 'perverse or abnormal sexual desire', though it was popularized outside psychology, as in the case of Mori Ōgai's 1909 novel Vita Sexualis.