The hymen is a thin piece of mucosal tissue that surrounds or partially covers the external vaginal opening. It forms part of the vulva, or external genitalia, and is similar in structure to the vagina.

In children, a common appearance of the hymen is crescent-shaped, although many shapes are possible. During puberty, estrogen causes the hymen to change in appearance and become very elastic. Normal variations of the post-pubertal hymen range from thin and stretchy to thick and somewhat rigid. Very rarely, it may be completely absent.

The hymen can rip or tear during first penetrative intercourse, which usually results in pain and, sometimes, mild temporary bleeding or spotting. Sources differ on how common tearing or bleeding after first intercourse are. The state of the hymen is not a reliable indicator of virginity, though "virginity testing" remains a common practice in some cultures, sometimes accompanied by surgical restoration of hymen to give the appearance of virginity. Minor injuries to the hymen may heal on their own, and not require surgical intervention.