Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Red-light district \Red-light district\ A district or neighborhood in which houses of prostitution are located; -- so called in allusion to the red light kept in front of many such resorts at night. [Colloq. or Cant]
Wiktionary
n. An area of a town where prostitution is concentrated.
WordNet
n. a district with many brothels
Wikipedia
A red-light district is a part of an urban area where a concentration of prostitution and sex-oriented businesses, such as sex shops, strip clubs, adult theaters, etc. are found. The term originates from the red lights that were used as signs of brothels. Areas in many big cities around the world have acquired an international reputation as red-light districts.
Usage examples of "red-light district".
Rimmer, strode down the main street of the red-light district, with Tonto following four or five paces behind.
There was no gambling, no small boys, no socializing, no nightclubbing, no mistress, no scuttling through the red-light district.
As the samurai in olden days had done he had disguised himself somewhat before setting out on his trek to the red-light district.