Source : Webster's Revised Unabridged Dictionary (1913)

Red-light district \Red-light district\
   A district or neighborhood in which disorderly resorts are
   frequent; -- so called in allusion to the red light kept in
   front of many such resorts at night. [Colloq. or Cant]

Source : WordNet®

red-light district
     n : a district with many brothels