A red-light district is a neighborhood where prostitution and other businesses in the sex industry flourish. The term "red-light district" was first recorded in the United States in 1894, in an article in The Sentinel, a newspaper in Milwaukee. Other mentions from the 1890s are numerous, and located all over the United States.

Some say the origin of the red light comes from the red lanterns carried by railway workers, which were left outside brothels when the workers entered, so that they could be quickly located for any needed train movement. Others speculate that the origin comes from the red paper lanterns that were hung outside brothels in ancient China to identify them as such. It was said that the lights were thought to be sensual. The color red has been associated with prostitution for millennia: in the Biblical story of Rahab, a prostitute in Jericho aided the spies of Joshua and identified her house with a scarlet rope. During World War I there were many brothels in Belgium and France; blue lights were used to indicate brothels for officers, red lights for other ranks.

One of the many terms used for a red-light district in Japanese is , literally meaning "red-line", apparently of independent origins from the English term. Japanese police drew a red line on maps to indicate the boundaries of legal red-light districts. They also have a term , meaning "blue-line", for a non-legal district. In different cultures red-light districts are identified differently, the most common being "district of prostitutes" (e.g., in Bengali - Khanki Para or Neighbourhood of Prostitutes.)

The Red Light District is the fifth album from Def Jam recording artist Ludacris. The album was released on December 6, 2004 and was eventually certified double platinum by the RIAA. It debuted at number one on the Billboard 200 album chart with 322,000 copies sold in its first week. The album's title means an area where prostitution is common: red-light district.