synoecism
The unification of towns, tribes etc. under one capital city or polis.
They always remained separate states and were never synoikised.
Where it comes from
From Ancient Greek συνοικισμός (sunoikismós), from συνοικίζειν (sunoikízein, “to make live with, to unite under one city”), from σύν (sún, “together”) + οἰκίζειν (oikízein, “to colonise”).
Adapted from the Wiktionary entry synoecism (CC BY-SA 4.0).