If you are a dissident, you are a person who is rebelling against a government. Dissidents can do their work peacefully or with violence.
Dissident is closely related to the word, dissent, which means objecting. People who are dissidents show their dissent. Catholic priests who advocate allowing women into the priesthood could be called dissidents, as could the Puritans who left England to live in colonial America. As an adjective, a dissident member of a group is one who disagrees with the majority of members.