un ‧ der ‧ world /ˈʌndəwɜːld $ ˈʌndərwɜːrld/ BrE AmE noun [singular]
[ Word Family: noun : ↑ world , ↑ underworld , ↑ worldliness ; adjective : ↑ world , ↑ worldly ≠ ↑ unworldly , ↑ worldwide ; adverb : ↑ worldwide ]
1 . the criminals in a particular place and the criminal activities they are involved in:
New York’s criminal underworld
2 . the Underworld the place where the spirits of the dead are believed to live, for example in ancient Greek stories