an electric railway operating below the surface of the ground (usually in a city)
"in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"
An underground railway.
"The new subway extension will finally connect the outer suburbs to the core of the metro, making commutes much faster for everyone living in that sprawling region."
a metropolitan area
In plain English: A metro is a fast train system that runs on tracks inside a city to help people get around quickly.
"The metro is usually less crowded during the early morning hours."
Usage: Use "metro" as a shorthand for a metropolitan area when referring to the urban region surrounding a major city, such as in the phrase "living in the metro." Avoid using it to describe a subway system unless you are specifically distinguishing that transit mode from other forms of public transport.
metropolitan
"The city council decided to expand the metro area's transit system to accommodate the growing population of suburbs surrounding downtown."
In plain English: Metro means something that is related to an underground train system used for getting around a city.
"The metro map on the wall shows all the subway lines in the city."
Usage: Use metro as an adjective only to mean "of or relating to a city and its suburbs," such as in metro area or metro station. Do not use it to describe something that is merely large or impressive, which is the incorrect usage of this abbreviation.
The word metro comes from the French clipping of métropolitain, which translates to "mother city" and originally referred to the Paris subway system. This term entered English as a direct translation of the name for the first part of the London Underground railway built in 1863.