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。

《牛津词典》

There is a red line in the heart, hampering your happiness with me.

心中有一条红线，牵制着你与我的幸福。

youdao

That is, by default a green line was covered and a red line was not covered.

也就是，默认条件下，绿色的行是覆盖的而红色的则不是覆盖的。

youdao

I constructed a series of home prices -that's the red line and the red line is -you can see how it's moved through history.

我绘制了一条房屋价格的时间序列曲线...,就是那条红色的线,你可以看到它的历史走势

耶鲁公开课 - 金融市场课程节选

Now when you want to maintain good health, high peaks, high spikes in blood glucose and the insulin response that provokes is not a very good thing, and so the red line is a far healthier way to be than the yellow line is.

如果你想保持健康,血糖和胰岛素都出现高峰值,这对健康来说可不是好事,红线要比黄线更健康

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选

"The Lib Dem leadership have decided that civil liberties is not a red line issue, " she said.

BBC: Lib Dems reject 'secret courts' plan at spring conference

"We are following the red line, sir, " the officer said, referring to a red line on the tarmac.

BBC: Obama limo breaks down in Israel after 'fuel mix-up'

"There is a red line for the government of Pakistan and one which we must respect, " he said.

BBC: Envoy damns US Afghan drug effort