Origins
The word 'metropolis' preserves one of the most evocative compound formations in Greek: 'mētropolis' (μητρόπολις), from 'mētēr' (μήτηρ, mother) and 'polis' (πόλις, city). A metropolis is literally a 'mother city.'
In the context of ancient Greek colonization, the term had a precise meaning. When a Greek city-state sent out colonists to found a new settlement, the founding city was called the 'mētropolis' of the colony. Corinth was the metropolis of Syracuse (founded c. 733 BCE). Miletus was the metropolis of dozens of colonies around the Black Sea. The relationship between metropolis and colony was significant: colonies maintained cultural, religious, and sometimes political ties to their mother city, and the metropolis took pride in its offspring. The bond was emotional as well as institutional — the colonists carried the sacred fire from the mother city's hearth to kindle the hearth of the new settlement.
The word's meaning shifted over time. In Hellenistic and Roman usage, 'metropolis' came to mean the chief city or capital of a region, regardless of colonial relationships. The most important city in a province — the administrative center, the largest and most influential — was its metropolis. This is the sense that Late Latin inherited and that English eventually adopted.
Latin Roots
In early Christian usage, 'metropolis' took on an ecclesiastical meaning. The bishop of the chief city in a Roman province acquired authority over other bishops in that province and was called a 'metropolitan.' The metropolitan's city was the ecclesiastical 'mother' of the surrounding dioceses, mirroring the ancient Greek relationship between mother city and colonies. The Eastern Orthodox Church still uses the title 'Metropolitan' for senior bishops.
In modern English, 'metropolis' has come to mean simply 'a very large city' — particularly one that serves as a cultural, economic, or political center. The derivative 'metropolitan' (adjective) describes anything relating to a large city or its surrounding region. A 'metropolitan area' includes the central city and its suburbs. The London Underground is called 'the Metropolitan Railway' (shortened to 'the Met' and eventually 'the Tube').
The Greek root 'mētēr' (mother) connects 'metropolis' to a broad Indo-European word family. The PIE root *méh₂tēr (mother) produced Greek 'mētēr,' Latin 'māter' (whence English 'maternal,' 'maternity,' 'matrix,' and 'matter'), Old English 'mōdor' (whence 'mother'), and cognates in virtually every Indo-European language. The root 'polis' connects it to the political word family: 'politics,' 'police,' 'policy,' 'cosmopolitan,' 'acropolis,' 'megalopolis,' and 'necropolis.'