A limit that when exceeded, or an action that when taken, cannot be reversed.(noun)
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Use "rubicon" in a sentence
"The original form of the game, piquet au cent, is now obsolete, having been displaced in the late nineteenth century by the present game, technically known as rubicon piquet."
"In his answer he describes the “constitutional rubicon” as"
"Wherever you stand on the Premier League's role in the endtimes, though, you have to concede Mark's emergence as a spokesmodel crosses a rubicon for sporting officialdom."