Term coined by French critics to describe a genre of film characterized by dark, somber tone and cynical, pessimistic mood. Typically such films portray the dark and gloomy underworld of crime and corruption with the heroes as well as the villains being cynical and disillusioned, trapped by their pasts and insecure or apathetic about the future. The genre is typifed by Hollywood black-and-white motion pictures of the 1940s and early 1950s, though later examples in color exist.

Source: Art & Architecture Thesaurus

http://vocab.getty.edu/page/aat/300263882