The Collaborative International Dictionary of English v.0.48:

Brothel \Broth"el\, n. [OE. brothel, brodel, brethel, a
   prostitute, a worthless fellow, fr. AS. ber['o][eth]an to
   ruin, destroy; cf. AS. bre['o]tan to break, and E. brittle.
   The term brothel house was confused with bordel brothel. CF.
   Bordel.]
   A house of lewdness or ill fame; a house frequented by
   prostitutes; a bawdyhouse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

brothel
    n 1: a building where prostitutes are available [syn:
         whorehouse, brothel, bordello, bagnio, house of
         prostitution, house of ill repute, bawdyhouse,
         cathouse, sporting house]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "brothel":
   Babylon, Gomorrah, Sodom, bagnio, bawdy house, bawdyhouse,
   bordello, cathouse, clip joint, crib, den, den of iniquity,
   den of thieves, den of vice, disorderly house, dive, dump,
   fancy house, fleshpots, gyp joint, harem, hellhole, hole,
   house of assignation, house of joy, house of prostitution, joint,
   lair, panel den, panel house, red-light district, seraglio, sink,
   sink of corruption, sink of iniquity, sporting house, stew, stews,
   tenderloin, whorehouse