D defs.my
Entry 1 sense Webster, 1913

Rubicon

/ro͞ob'-ĭk-ôn/ · Ru·bi·con · IPA /ˈɹubəˌkɑn/
01 prop. n. A small river which separated Italy from Cisalpine Gaul, the province alloted to Julius Caesar.
  1. 1.
    A small river which separated Italy from Cisalpine Gaul, the province alloted to Julius Caesar.(Anc. geog.)