a system of public transport consisting of trains that travel in tunnels below a city. The Underground in London is also called the Tube and in some other cities it is also called the Metro. The American word is subway.
Synonyms and related words
Types of train or railway system
Amtrak
boat train
bullet train
2
a group of people who take part in secret and usually illegal activities, especially against a government
Synonyms and related words
Protesters, rebels and terrorists
activist
agitator
anarchist
the Underground
noun
US /ˈʌndərˌɡraʊnd/
Other entries for this word
the Underground Railroad
DEFINITIONS2
1
a group of people who take part in secret and usually illegal activities, especially against a government