Municipium (pl. municipia) is the Latin term for a town or city. Etymologically the municipium was a social contract among municipes, the "duty holders", or citizens of the town. The duties, or munera, were a communal obligation assumed by the municipes in exchange for the privileges and protections of citizenship. Every citizen was a municeps.
The distinction of municipia was not made in the Roman kingdom; instead, the immediate neighbors of the city were invited or compelled to transfer their populations to the urban structure of Rome, where they took up residence in neighborhoods and became Romans per se. Under the Roman Republic the practical considerations of incorporating communities into the city-state of Rome forced the Romans to devise the concept of municipium, a distinct state under the jurisdiction of Rome. It was necessary to distinguish various types of municipia and other settlements, such as the colony. In the early Roman Empire these distinctions began to disappear; for example, when Pliny the Elder served in the Roman army, the distinctions were only nominal. In the final stage of development, all citizens of all cities and towns throughout the empire were equally citizens of Rome. The municipium then simply meant municipality, the lowest level of local government.