underground
n. countablen. a system of trains that travel in tunnels under a city. You use this to travel quickly without dealing with street traffic.
n. an electric railway system operating below ground level in a large city. Often used with a definite article to refer to the specific network in cities like London.
I take the underground to work every morning.
The city's underground is the fastest way to travel between the museum and the main square during rush hour.
Urban planners argue that expanding the underground is essential for reducing the carbon footprint of the city's daily commuters.
From Middle English undergrounde (adverb), equivalent to under + ground or under- + ground. Compare Dutch ondergrond, ondergronds, German Untergrund, Danish undergrunds.
See the etymology of the corresponding lemma form.
Usually preceded by 'the' when referring to the transport system.
I am going by the subway in LondonI am going by the underground in LondonIn British English, 'underground' is the standard term for the rail system, while 'subway' usually refers to a pedestrian tunnel.