What is Brothel?

Legal Definition
A brothel or bordello is a place where people may come to engage in sexual activity with a prostitute, sometimes referred to as a sex worker. Technically, any premises where prostitution commonly takes place qualifies as a brothel. However, for legal or cultural reasons, establishments sometimes describe themselves as massage parlors, bars, strip clubs, body rub parlours, studios or by some other description. Sex work in a brothel is considered safer than street prostitution.
-- Wikipedia
Legal Definition
Crim. law. Bawdy-houses, the common habitations of prostitutes; such places have always been deemed common nuisances in the United States, and the keepers of them may be fined and imprisoned.

2. Till the time of Henry VIII, they were licensed in England, when that lascivious prince suppressed them. Vide 2 Inst. 205, 6; for the history of these pernicious places, see Merl. Rep. mot Bordel Parent Duchatellet, De la Prostitution dans la ville de Paris, c. 5, §1; Histoire de la Legislation sur les femmes publiques, & c., par M. Sabatier.
-- Bouviers Law Dictionary
Legal Definition
A bawdy-house; a house of ill fame; a common habitation of prostitutes.
-- Black's Law Dictionary