Invulnerable means impossible to hurt, damage, or defeat. When something is invulnerable, it cannot be harmed no matter what attacks or challenges come its way.

In Greek mythology, the hero Achilles was supposedly invulnerable except for one spot on his heel. Superman is invulnerable to bullets, which bounce right off him. A castle with thick stone walls and a deep moat might seem invulnerable to attackers.

People use it both for physical protection and for other kinds of harm. A dictator might feel politically invulnerable if nobody dares challenge his power. A student who has studied every possible question might feel invulnerable going into a test.

But true invulnerability is rare. Even the strongest fortress has weaknesses, and Achilles learned that being almost invulnerable wasn't enough. The word often appears when someone discovers they aren't as invulnerable as they thought. That overconfident student might have felt invulnerable until the teacher asked an unexpected question.