An underground film is a film that is out of the mainstream either in its style, genre or financing.
Notable examples include: John Waters' Pink Flamingos, David Lynch's Eraserhead, Andy Warhol's Blue Movie, Rosa von Praunheim's Tally Brown, New York, Frank Henenlotter's Basket Case, Nikos Nikolaidis' Singapore Sling, Rinse Dreams' Café Flesh, and Jörg Buttgereit's Nekromantik.