A civil action is an action brought by a plaintiff to enforce or protect private rights.
Put another way, a civil action is a legal demand instituted in a court to enforce a right. See Black's Law Dictionary 26 (5th ed. 1979) ("Action" means "the legal and formal demand of one's right from another person or party made and insisted on in a court of justice."); William C. Anderson, A Dictionary of Law 26 (1996) ("Civil action" seeks to "recover a private right or compensation for deprivation thereof."); Frederic J. Stimson, Glossary of Technical Terms, Phrases, and Maxims of the Common Law 7 (1881) ("Action" means "the legal demand of one's right"; "civil action" is an action "to enforce a private right.").