Synoecism or synecism (/sɪˈniːsɪzəm/ si-NEE-siz-əm; Ancient Greek: συνοικισμóς, sunoikismos, Ancient Greek: [syːnɔi̯kismós]), also spelled synoikism (/sɪˈnɔɪkɪzəm/ 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. A dioecism was undertaken for similar reasons, such as the settling of new and independent communities within territory of Constantinople abandoned due to a contraction of population, or the contraction of Thessaloniki out of its former neighborhoods outside the city walls due to the occupation of the countryside by the Turks.