1) An unauthorized, mock court or legal proceeding, e.g. a tribunal of sorority sisters created to settle disputes within the sorority, in which some or all of the accused's due process rights are ignored and the outcome appears to be predetermined. The term's first known use was in the American West in the 1850s.
2) Also slang for an authorized court or legal proceeding in which fair proceedings are impossible due, for example, to a partial judge or excessive press coverage.
See, e.g. Skilling v. United States, 130 S.Ct. 2896, 2914–17 (2010) and Rideau v. Louisiana, 373 U.S. 723, 726 (1963).