blue line / (historical) Short for 青線地帯(あおせんちたい) (aosenchitai): Aosen (districts in Japan where prostitution and the sex industry flourished)

red line / (historical) Short for 赤線地帯(あかせんちたい) (akasenchitai): Akasen (districts in Japan where prostitution and the sex industry flourished)

poverty line

the brink of starving to death

(mathematics) asymptote

Synonym of X線 (ekkusu-sen, “X-ray”); a Röntgen ray

(physics) world line

(science fiction) mysterious ray / beam

drawn line

(geography) Arctic Circle (one of the five major circles of latitude)