red line; line in the sand
Figurative boundary that must not be crossed; often used in political, diplomatic, or personal contexts.
That remark was a red line he could not allow to be crossed.
red line (ice hockey); center line
Sports term: the red center line in ice hockey.
アイスホッケーでは、レッドラインを越えてパスを出すと反則になる。
In ice hockey, passing across the red line is a violation.