(Oxford English Dictionary)
Etymology: < classical Latin mētropolis capital city (from late 2nd cent. in inscriptions), see of a metropolitan bishop (4th cent. in post-classical Latin) and its etymon ancient Greek μητρόπολις mother city of a colony, capital city
1. Christian Church. The seat or see of a metropolitan bishop. Now chiefly hist. exc. in Orthodox Church.
2. orig. Greek Hist. The mother city or parent state of a colony.
3a. The chief town or city of a country (occas. of a province or district), esp. the one which is the seat of government; a capital. In extended use: any large, bustling city.
[For Postcolonial studies, metropolis typically refers to an urban center in the colonizing country to which colonized intellectuals go for modern studies or professional experience.]