The porn video tag "hentai-masturbation" refers to a specific type of adult content that involves masturbation within the context of hentai. Hentai is a genre of adult Japanese anime and manga, often characterized by explicit sexual content and fantastical themes or characters. In this case, the tag specifically denotes a scene where a character (or characters) engaged in self-stimulation for the purpose of achieving sexual pleasure. The term "hentai" comes from the Japanese word for perversion or abnormality, and is often used to describe pornographic or erotic content within that culture.