noun
an issue or a demand that one person or group refuses to change their opinion about during a disagreement or negotiations :
The issue of sovereignty is a red line that cannot be crossed.
noun
an issue or a demand that one person or group refuses to change their opinion about during a disagreement or negotiations :
The issue of sovereignty is a red line that cannot be crossed.
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005