noun
blue-line district; area of illegal prostitution
Historical term for areas where illegal prostitution occurred, marked by a blue line on police maps from the 1940s until 1956, when the Anti-Prostitution Law was enacted.
戦後の東京には、青線地帯と呼ばれる地域がいくつか存在した。
In postwar Tokyo, there were several areas known as blue-line districts.
The name 'blue-line district' came from the fact that these areas were enclosed by a blue line on police maps.