red line

(This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium.)

red line (plural red lines)

1. (figuratively) A boundary or limit which should not be crossed.
2. (ice hockey) The thick red line on the ice which divides the rink in half.

• blue line
• goal line (“thin red lines in front of the goals”)
• green line
• redline (verb)
• single yellow line

• “red line”, in Collins English Dictionary.
• “red line”, in Merriam-Webster Online Dictionary
• “red line” in the Cambridge English Dictionary, Cambridge: Cambridge University Press.

• Nile red, relined