red-light district 
noun
An area of a town or city containing many brothels, strip clubs, and other sex businesses.This is even more the case in a red-light district than in other urban public areas....
- The car was parked in a relatively small alleyway in the seedier part of the city; near the red-light district.
- Occupying several blocks on two of the most beautiful canals in the very centre of the city, the red-light district is pretty much unavoidable.
Origin
Late 19th century: from the use of a red light as the sign of a brothel.