青線あおせん

1. 1blue line Translated by Jiyomi
2. 2area around a red-light district where illegal prostitution occurred (marked on police maps by a blue line; 1940s-1956) Translated by Jiyomi
3. 3non-legal public waterway (denoted by a blue line on cadastral maps; sometimes no longer physically existing) Translated by Jiyomi

Kanji · tap to open

JMdict entry 2829931. Source and licence.