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

But most of them now see attacks on energy infrastructure as a red line.

From BBC • Apr. 6, 2026

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 • Mar. 11, 2026

Pahlavi accused them of threatening national unity and said "Iran's territorial integrity is the ultimate red line."

From Barron's • Mar. 5, 2026

He seemed to suggest that this was another red line.

From BBC • Feb. 25, 2026

“So when DC’s driving along, a black line tells it to do something, and a red line will tell it to do something else. Watch.”

From "A High Five for Glenn Burke" by Phil Bildner