Rubicon

Rubicon, a famous river of Italy, associated with Julius Cæsar, now identified with the modern Fiumecino, a mountain torrent which springs out of the eastern flank of the Apennines and enters the Adriatic N. of Ariminum; marked the boundary line between Roman Italy and Cisalpine Gaul, a province administered by Cæsar; when he crossed it in 49 B.C. it was tantamount to a declaration of war against the Republic, hence the expression “to cross the Rubicon” is applied to the decisive step in any adventurous undertaking.

Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)