Rubicon n., pl. Rubicons [L. Rubico, a stream south of Ravenna, Italy.] 1. Roman History. The stream which, in the Republican era of ancient Roman history, served as a boundary between Italy and Cisalpine Gaul. Since a Roman general or governor was forbidden to lead his troops in arms across a Roman boundary into Italy, Julius Caesar’s crossing of the Rubicon in 49 B.C. meant that he was irrevocably committed to war against Pompey and the senatorial party. 2. A boundary, line or limit whose crossing indicates irrevocable commitment to a decision or a complete break with the past or that the “die is cast.” a) The Prime Minister after considerable deliberation and hesitation, crossed the Rubicon by tendering his letter of resignation. b) But in less than 25 years he has crossed an American Rubicon from poor immigrant quarters to the comfortable suites of upper U.S. management as a group vice president… (Newsweek Int. May 4, 1981:37). See alea jacta est and jacta alea est.
From Word Dictionary of Foreign Expressions