What is a rubicon?

The Rubicon is a river in Northern Italy that famously marked the boundary between the Roman Republic and the Roman Empire in 49 BCE. The crossing of the Rubicon by Julius Caesar with his armies is celebrated as an act of rebellion against the Roman Senate and the beginning of the Civil War. The phrase "crossing the Rubicon" has since come to mean a point of no return or a decisive moment in history. The Rubicon is also a term used to describe a difficult or challenging decision or situation that brings a fundamental change to the direction or trajectory of someone's life or career.