Rubicon

Rubicon (r`bĭkŏn), Lat. Rubico, small stream that flows into the Adriatic and in Roman times marked the boundary between Cisalpine Gaul and ancient Italy. In 49 B.C., after some hesitation, Julius Caesar crossed the Rubicon to march against Pompey in defiance of the senate's orders. He thus committed himself to conquer or to perish, and "to cross the Rubicon" now means to take an irrevocable step.

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Rubicon

Small stream that separated Cisalpine Gaul from Italy in the era of the Roman republic. The movement of Julius Caesar's forces over the Rubicon into Italy in 49 BC violated the law that forbade a general to lead an army out of the province to which he was assigned. Caesar's act thus amounted to a declaration of war against the Roman Senate and resulted in the three-year civil war that left Caesar ruler of the Roman world. “Crossing the Rubicon” became a popular phrase describing a step that irrevocably commits a person to a given course of action.

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Rubicon

1. a stream in N Italy: in ancient times the boundary between Italy and Cisalpine Gaul. By leading his army across it and marching on Rome in 49 bc, Julius Caesar broke the law that a general might not lead an army out of the province to which he was posted and so committed himself to civil war with the senatorial party

2. a penalty in piquet by which the score of a player who fails to reach 100 points in six hands is added to his opponent's

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Rubicon 

a river on the Italian peninsula, emptying into the Adriatic Sea, north of the city of Rimini. Until 42 B.C. it served as the border between Italy and the Roman province of Cisalpine Gaul. On Jan. 10, 49 B.C., Julius Caesar and his army illegally (as proconsul, he was entitled to head the army only outside Italy) crossed the Rubicon and invaded Italy, thus initiating civil war. Hence, the well-known expression “to cross the Rubicon,” meaning to make an irrevocable decision.