red line
noun/ˌred ˈlaɪn/
/ˌred ˈlaɪn/
- an issue or a demand that one person or group refuses to change their opinion about during a disagreement or negotiations
（争论或谈判中）拒绝改变立场的问题（或要求） - The issue of sovereignty is a red line that cannot be crossed.
主权问题是一条不能跨越的红线。
- The issue of sovereignty is a red line that cannot be crossed.