Pronunciation: (rOO'bi-kon"),[key] —n. 1. a river in N Italy flowing E into the Adriatic. 15 mi. (24 km) long: in crossing this ancient boundary between Cisalpine Gaul and Italy, to march against Pompey in 49b.c., Julius Caesar made a major military commitment.
2. cross or pass the Rubicon, to take a decisive, irrevocable step: Our entry into the war made us cross the Rubicon and abandon isolationism forever.