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)

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)

poverty line

the brink of starving to death

(mathematics) asymptote

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

(attributive, geometry) collinearity

copper wire

reticle; crosshair

(acoustics, physics) a sound ray, an acoustic ray