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

"Nuclear infrastructure has generally been treated as a red line in the region. The message is: no strategic infrastructure in the Gulf is completely immune."

From Barron's • May 18, 2026

That red line is now being crossed for many other people.

From Salon • Apr. 28, 2026

“Disrupting order and safety in the Strait of Hormuz is considered a red line for Iran,” the Iranian Navy Command said in a statement.

From Los Angeles Times • Apr. 22, 2026

“Territorial integrity is a red line for most Iranians, whether they support the regime or oppose it,” she said.

From The Wall Street Journal • Apr. 3, 2026

Or a red line and a blue line.

From "The Woman All Spies Fear" by Amy Butler Greenfield