Definition of: Rubicon

(r′bi·kon)
A river in north central Italy, flowing 15 miles NE to the Adriatic: modern Fiumicino; it formed the boundary separating Caesar's province of Gaul from Italy, and by crossing it under arms he committed himself to a civil war with the Roman government then controlled by Pompey; hence, to cross the Rubicon, to be committed definitely to some course of action; make an irrevocable move.