(meteorology) isotherm

ground (earth) wire; earthed line

Kyūjitai form of 経線 (“longitude”)

(rail transport) circle route; loop line

(geometry, engineering) extended line (usually as an auxiliary line) / extension of a traffic route (e.g. highway or railway line) / (figuratively) extension or result developed from an existing thing

(technology) Synonym of アンテナ (antena, “antenna”); an aerial (device for receiving or transmitting)

borders (outer lines) of a box

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