A metropolis (from the Greek μήτηρ, mētēr meaning 'mother' and πόλις, pólis meaning 'city/town') is a big city, in most cases with over half a million inhabitants in the city proper, and with a population of at least one million living in its urban agglomeration. Big cities belonging to a larger urban agglomeration, but which are not the core of that agglomeration, are not generally considered a metropolis but a part of it. A metropolis is usually a significant economical, political and cultural center for some country or region, and an important hub for regional or international connections and communications. The plural of the word is most commonly metropolises, though metropoleis is sometimes used as well.

In a broader sense, it refers to the city or state of origin of a colony (as of ancient Greece), a city regarded as a center of a specified activity, or a large important city.

Source: Wikipedia