The Collaborative International Dictionary of English v.0.48:

Rubicon \Ru"bi*con\, prop. n. (Anc. geog.)
   A small river which separated Italy from Cisalpine Gaul, the
   province alloted to Julius Caesar.
   [1913 Webster]

   Note: 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]

WordNet (r) 3.0 (2006):

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: Rubicon, point of
       no return]