A nickname is a special name that a person may be called that is different from their real name or from their birth name. It is not a legal thing; it is a social thing. The name may be given to them by their friends or family. It may be like the person's real name or completely different. Nicknames often come from how a person looks, or from something they are especially known for doing. They can also be a shortened version of the given name.
Nicknames can also be unkind and rude, and used by people who dislike the person given the nickname. Often famous people are given nicknames by the writers of newspapers or other people in the media, such as TV.