red line

American  

noun

1. Ice Hockey. a line of the color red that is parallel to and equidistant from the goal lines and divides the rink in half.

2. a limit, point, or boundary beyond which one may not go without incurring negative consequences.

   The use of chemical weapons is a red line, which, if crossed, could trigger military action.

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red line British  

noun

1. a point beyond which a person or group is not prepared to negotiate

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Etymology

Origin of red line

First recorded in 1960–65

Example Sentences

Pakistan's president said that this crossed a red line.

From BBC

"A red line has been crossed", one Gulf official told me.

From BBC

There’s also the matter of the three tops marked in the chart above by the red lines.

From MarketWatch

This is seemingly a red line for Anthropic, which doesn’t want its products used for autonomous weapons or mass surveillance.

From The Wall Street Journal

Yet US interest in the vast Arctic territory has not waned, and the red line on the Greenlandic -- and Danish -- side remains the same: any transfer of sovereignty is off the table.

From Barron's