They called it Metro thinking it would be easier to pronounce and spell than Metropolitan, which is what all the other merged cities use for their formal title. Only Louisville pushes the envelop to put the slang word Metro in official city documents and on vehicles, even though the dictionary says it refers to a subway. metro [me-troh] noun, plural met·ros. 1. the underground electric railway of Paris, France, Montreal, Canada, Washington, D.C., and other cities. 2. subway