Definition of Rubicon noun from the Oxford Advanced Learner's Dictionary

 

Rubicon

 noun
noun
BrE BrE//ˈruːbɪkən//
 
; NAmE NAmE//ˈruːbɪkɑːn//
 
the Rubicon[singular]
 
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the point at which 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 which formed the border between Italy and Gaul. When Julius Caesar broke the law by crossing it with his army in 49BC, it led inevitably to war.
See the Oxford Advanced American Dictionary entry: Rubicon