The literal meaning in
Greek is something like "making the homes be together".
In Greek history this term marks the process through which vilages and small settelments united to create the Greek poleis in the end of the Greek Dark Age.
In every polis the Synoikistes (the person who, accroding to myth performed the synoikismos) won godlike worship.
The mythic Synoikistes of Attica (though the peninsula probably never went through synoikismos in actual history), is Theseus.