Rubicon

Concise Oxford English Dictionary © 2008 Oxford University Press:
Rubicon /ˈruːbɪk(ə)n, -kɒn/

▶noun

• 1 a point of no return.

• 2 (in piquet) an act of winning a game against an opponent whose total score is less than 100.

– origin the name of a stream in NE Italy marking the ancient boundary between Italy and Cisalpine Gaul, which Julius Caesar crossed in 49 bc, breaking the law and so committing himself to war.