Traditionally a virgin is a person that had not had sexual intercourse. Usually, the idea of virginity is used for women, but it is also used for men who have not had sexual intercourse.
Virgin women usually have an unbroken hymen, but this is not always the case. A hymen is human tissue that blocks the opening of the vagina about two inches deep. When a man inserts his penis or other object(s) are inserted into the vagina, it usually tears or breaks the hymen which results in bleeding.
Hence when vaginal sexual intercourse has already occurred, a woman has been penetrated and the hymen is broken, the virginity is lost. This is irreversible and cannot be regained. Hence, the best and only way to retain virginity is to abstain from Sexual intercourse i.e vaginal penetration.