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.

---

red line British  

noun

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

---

Etymology

Origin of red line

First recorded in 1960–65

Example Sentences

So far, Sheinbaum’s concessions haven’t crossed a red line for López Obrador, who is writing a book at his country home in Palenque in southern Mexico.

From The Wall Street Journal • Apr. 28, 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

The episode exposed a fault line in Silicon Valley between engineers who viewed autonomous targeting as an ethical red line and defense officials who saw it as essential.

From Barron's • Apr. 5, 2026

If you got four Quarter-Stars, a red line was drawn through the four dots indicating that you had completed your Star.

From "Boy: Tales of a Childhood" by Roald Dahl