Ru'bi-con, n.

n. (Anc. Geog.) A small river which separated Italy from Cisalpine Gaul, the province allotted to Julius Cæsar.

☞ When Cæsar undertook the conquest of Italy, the act of crossing this river was the first and significant step of the enterprise. Hence the phrase to pass the Rubicon signifies to take the decisive step, by which one is committed to a hazardous or difficult enterprise.