Underworld/ˈʌndərwɜːrld/ Definition 1 of 2Pro Tip 1/3Criminal Context Used to talk about crime, especially when involving organized groups like the mafia. He wrote a book on the New York underworld. Pro Tip 2/3Negative Connotation 'Underworld' often has a dark, dangerous feel, depicting a hidden criminal society. The film noir explored the city's seedy underworld. Pro Tip 3/3Hidden Operations Refers to secret operations, not visible to law enforcement or the public. Undercover agents infiltrate the underworld to gather intelligence.