Definition of Rubicon noun from the Oxford Advanced American Dictionary

 

Rubicon

 noun
noun
NAmE//ˈrubɪˌkɑn//
 
the Rubicon[singular]
 
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the point at that a decision has been taken which can no longer be changed Today we cross the Rubicon. There is no going back. From the Rubicon, a stream that formed the border between Italy and Gaul. When Julius Caesar broke the law by crossing it with his army in 49B.C., it led inevitably to war.
See the Oxford Advanced Learner's Dictionary entry: Rubicon