The term NOT VULNERABLE (TO) can be summarized as being IMMUNE or invulnerable. The crossword solution for this term is IMMUNE, which is a 6 letter word. Being IMMUNE means that one is not susceptible to a particular disease or other negative influence. It is often used in the context of medical immunity, where a person is protected from a virus or bacteria due to a prior exposure or vaccination. In other contexts, it can refer to being resistant to emotional or psychological harm.