In the history of Rome, the Latin term civitas (plural civitates), according to Cicero in the time of the late Roman Republic, was the social body of the cives, or citizens, united by law (concilium coetusque hominum jure sociati). It is the law that binds them together, giving them responsibilities (munera) on the one hand and rights of citizenship on the other. The agreement (concilium) has a life of its own, creating a res publica or "public entity" (synonymous with civitas), into which individuals are born or accepted, and from which they die or are ejected. The civitas is not just the collective body of all the citizens, it is the contract binding them all together, because of which each is a civis.

Civitas is an abstract formed from civis. Nicolet traces the first word and concept for the citizen at Rome to the first known instance resulting from the synoecism of Romans and Sabines presented in the legends of the Roman Kingdom. According to Livy the two peoples participated in a ceremony of union after which they were named Quirites after the Sabine town of Cures. The two groups became the first curiae, subordinate assemblies, from co-viria ("fellow assemblymen," where vir is "man," as only men participated in government.) The Quirites were the co-viri. The two peoples had acquired one status. The Latin for the Sabine Quirites was cives, which in one analysis came from the Indo-European *kei-, "lie down" in the sense of incumbent, member of the same house. City, civic, civil all come from this root. Two peoples were now under the same roof, so to speak.