means A city's underground or rapid-transit rail system — or, loosely, the dense urban region it serves.

from Short for "metropolitan," itself from the Greek metropolis — mētēr ("mother") plus polis ("city"), literally the "mother city" that founded colonies. The clipped form took off with the Paris underground, the Chemin de Fer Métropolitain, opened around 1900 and quickly nicknamed le métro; the word then rode the rails into cities worldwide.