Source : Webster's Revised Unabridged Dictionary (1913)

Rubicon \Ru"bi*con\, n. (Anc. geog.)
   A small river which separated Italy from Cisalpine Gaul, the
   province alloted to Julius C[ae]sar.

   Note: By leading an army across this river, contrary to the
         prohibition of the civil government at Rome, C[ae]sar
         precipitated the civil war which resulted in the death
         of Pompey and the overthrow of the senate; hence, the
         phrase to pass 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.

Source : WordNet®

Rubicon
     n 1: the boundary in ancient times between Italy and Gaul;
          Caesar's crossing it with his army in 49 BC was an act
          of war
     2: a line that when crossed permits of no return and typically
        results in irrevocable commitment [syn: {point of no
        return}]