Vulnerability is the quality of being easily hurt or attacked. Some seniors think it's funny to pick on the ninth graders because of their vulnerability.
Vulnerability comes from the Latin word for "wound," vulnus. Vulnerability is the state of being open to injury, or appearing as if you are. It might be emotional, like admitting that you're in love with someone who might only like you as a friend, or it can be literal, like the vulnerability of a soccer goal that's unprotected by any defensive players.