Synoecism or synecism ( si-NEE-siz-əm; Ancient Greek: συνοικισμóς, sunoikismos, Ancient Greek: [syːnɔi̯kismós]), also spelled synoikism ( si-NOY-kiz-əm), was originally the amalgamation of villages in Ancient Greece into poleis, or city-states. Etymologically the word means "dwelling together (syn) in the same house (oikos)." Subsequently, any act of civic union between polities of any size was described by the word synoikismos. The closest analogy today is the incorporation of a city; in fact, "incorporation" is often used to translate synoikismos, in addition to the Latinized synoecism. Synoecism is opposed to Greek dioecism (διοικισμóς, dioikismos), the creation of independent communities within the territory of a polis.

Synoecism is the result of a few major factors, mainly an increase in population density of adjacent settlements, with an incorporation proposed for economic, political or ideological advantages, such as the synoecism of the communities of Attica into Athens, or by imposition of a ruling power, such as the synoecism of Messenia into the newly built city of Messene. Additionally, synoecism may be the result of less active forces which often take the form of shared cultural elements such as religion or language.

Although there were differences between synoecism in ancient Greece and ancient Rome, the same general concept is deduced from the history of both civilizations. Before political union, the future (combined) population of the polity constituted smaller settlements that were not obligated to each other, or at least not by the contract that was later to institute their political union. A settlement or group of settlements might be constituents of another polity from which they would be annexed or transferred.