red line

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

1960–65 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
red line
a point beyond which a person or group is not prepared to negotiate

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
But then he took his pen and drew a red line clear through it.
And yet it's a red line, so you're not even supposed to bring it up.
As he walks, the map adjusts to each new point of view, and a red line marks
  the sidewalk leading to his destination.
The output from the telescope appeared on four three-track pen recorders as a
  squiggly red line on moving chart paper.
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