Have you ever seen a picture of a protester who is about to be carried off by police but is still shouting or resisting, fighting to the bitter end? That person is defiant.
Someone who is defiant is bold, even in the face of defeat. A defiant person is usually fighting a powerful enemy. People who protest in countries controlled by dictators are defiant. Rosa Parks was defiant when she refused to give up her seat on the bus, even though the law at that time dictated that black people had to stand when whites needed a seat. Small children are defiant too, sometimes over nothing more than a request to share their toys.