黑社会 is the general Chinese term for organized crime. In Hong Kong, the specific term 三合会 (sānhéhuì, Triad) is used for the historically rooted secret societies. Many iconic Hong Kong films like 'Infernal Affairs' depict 黑社会.
usage
黑 (black) often implies 'illegal' or 'underground' in Chinese: 黑市 (hēishì, black market), 黑钱 (hēiqián, dirty money), 黑客 (hēikè, hacker).