Etymology

The term metro for underground rapid transit originally comes from the name of the Paris underground (Chemin de Fer Métropolitain) which was shortened to métropolitain "metropolitan" and then to métro. The prefix metro comes from the word metropolis from the Greek μήτηρ ( mētēr) "mother" and πόλις (pólis) "city/town" which means is a big city. The term was originally used by the Paris rapid transit system to differentiate it from non-urban railway systems.

Noun

1. An underground railway other than those in the US or London, in particular that in Paris.
2. A train that runs on such an underground railway.
3. An urban rapid transit light railway
4. A train that runs on such a railway.

Synonyms

el/El (US), subway (US), Tube (the London underground), underground (UK), underground railway (UK)