Webster's Dictionary 
(n.) A small river which separated Italy from Cisalpine Gaul, the province alloted to Julius Caesar.
The Nuttall Encyclopedia 
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.