A metropolis or metropolitan archdiocese is a see or city whose bishop is the metropolitan of a province. Metropolises, historically, have been important cities in their provinces.
In the Eastern Orthodox Churches, a metropolis (also called metropolia or metropolitanate) is a type of diocese, along with eparchies, exarchates, and archdioceses; and it may be an ancient diocese (especially in Byzantine areas) or a set of dioceses. A similar system exists in some Eastern Catholic Churches.