The Collaborative International Dictionary of English v.0.48:

Brassiere \Bras`siere"\ Brassiere
\Bras`si[`e]re"\(br[.a]*z[=e]r"), n. [F.]
   A form of woman's undergarment, often stiffened with wire or
   whalebones, or the like, and worn to cover and support the
   breasts; -- also called bra. It usually has straps which
   support it from the shoulders, but strapless variants are
   also made.

   Syn: bra, bandeau.
        [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

brassiere
    n 1: an undergarment worn by women to support their breasts
         [syn: brassiere, bra, bandeau]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "brassiere":
   advocate, alpenstock, arm, athletic supporter, back, backbone,
   backing, bandeau, bearer, bra, brace, bracer, bracket, buttress,
   cane, carrier, cervix, corset, crook, crutch, falsies,
   foundation garment, fulcrum, girdle, guy, guywire, jock, jockstrap,
   mainstay, maintainer, mast, neck, prop, reinforce, reinforcement,
   reinforcer, rest, resting place, rigging, shoulder, shroud,
   soutien-gorge, spine, sprit, staff, standing rigging, stave, stay,
   stick, stiffener, strengthener, support, supporter, sustainer,
   upholder, walking stick