UNDERGROUND - [noun] a mean of transport in which trains run under the surface of the ground
Examples of use
Underground in London and Madrid used to be a target of terrorists, because a lot of people use it.
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English speaking practise
Remember that if you don't use that word in English conversations or writing you'll forget it in a short time. So the best thing you can do to learn this word is just trying to use it in speaking or writing. Remember also that practise is the key to fluent english speaking.
You can always find a partner, who would also like to improve his/her