Akasen

 was the Japanese slang term for districts historically engaged in the sex work industry in Japan, specifically within the time period of January 1946 through to March 1958.
Etymology.
 literally translates as "red-line". Though similar to another term previously used for red-light districts, , was used as a collective term for red-light districts only between 1946 and 1958, following an issue ordered by GHQ (SCAPIN 642) nationwide to abolish Japan's legalised system of sex work.
Another term, , literally "blue-line", was used for "non-permitted" or "non-legal" sex industry districts. In Tokyo, the area directly across the river from , now called ) was a well-known district; it features in some of 's short stories.
The term is often compared directly with the term "red-light district" in the west. However, this does not explain why the counterpart "non-permitted districts" were known as (blue-line) districts. In practice, and referred to the colors on municipal zoning maps that outlined brothel districts () and "normal" entertainment districts ().
History.
The precusor of districts were , legal red-light districts in Japan where both brothels and sex workers (known collectively as , the higher ranks of which were known as ) recognised by the Japanese government operated. In January 1946, GHQ issued an order (SCAPIN 642) nationwide to abolish this licensed sex work system. This had a number of impacts on areas, the largest of which was the being renamed as districts. Brothels had to rename themselves as either or , sex workers were no longer bound by state-guaranteed contracts, and all known houses of sex work were declared to have "Off Limits" status by SCAP GHQ. The order also had the effect of disbanding the short-lived Recreation and Amusement Association, which had, for a period of one year, worked to ensure that sex workers were not abused and exploited by stationed American soldiers, amongst other things.
Despite the increased restrictions and the restructuring of the sex work industry, commercial brothels continued to operate within the law, with districts remaining the designated regions for state-regulated sex work. Due to GHQ orders, brothels - often numbering in their hundreds - began to front non-adult faces of their businesses (such as coffee shops, cafés and beer halls), but would offer sexual services to customers, creating new avenues for the sex industry to continue, especially in popular districts such as , the region of Tokyo.
However, following the partial disbanding of traditional red-light areas, formerly law-abiding sex work businesses began operating within areas, creating difficulties for the SCAP to identify brothels and continue its "Off Limits" policy.
In 1958, the was enforced, thus officially abolishing legalised sex work, the red-light districts and their label of .
Despite the criminalisation of sex work, by the beginning of the 21st century, businesses such as and massage parlours had come into existence, regulated under the Act, also known as or . These businesses, which avoid criminalisation through offering only non-coital sex acts, are required to file a license application for permission to abide by the to remain in operation.