defloration
Not to be confused with menstruation
The tearing of a girl's hymen, indicating she has lost her virginity.
In hentai, this is typically indicated by blood coming from the vagina during or after sex, implying that the hymen has been torn and the girl has lost her virginity. In real life, the hymen is not always torn by first sexual intercourse and thus is not a reliable indicator of virginity.
Use blood from vagina instead if the blood comes from an injury such as vaginal tearing rather than loss of virginity.