prop. n. (Anc. geog.) A small river which separated Italy from Cisalpine Gaul, the province alloted to Julius Caesar. [ 1913 Webster ]

☞ By leading an army across this river, contrary to the prohibition of the civil government at Rome, Caesar precipitated the civil war which resulted in the death of Pompey and the overthrow of the senate; hence, the phrase
to pass the Rubicon or
cross the Rubicon signifies to take the decisive step by which one is committed to a hazardous enterprise from which there is no retreat. [ 1913 Webster ]