What it is- being sexually inexperienced; a social construct (not a physical thing); however you wish to define it for yourself.
Someone's virginity, or lack of, has no hold over their worth as a human being.
You can have oral sex and still be a virgin. You can dry-hump (rubbing you genitals with your partners, usually through clothing with no penetration), and still be a virgin. You can have anal sex and still be a virgin. You can define your virginity however you want.
*You can still consider yourself a virgin if you've been sexually assulted.* That is entirely your choice and it's one that can't be taken from you. No one else can define your virginity.