Source : Webster's Revised Unabridged Dictionary (1913)

Thorax \Tho"rax\, n. [L., fr. Gr. ?.]
   1. (Anat.) The part of the trunk between the neck and the
      abdomen, containing that part of the body cavity the walls
      of which are supported by the dorsal vertebr[ae], the
      ribs, and the sternum, and which the heart and lungs are
      situated; the chest.

   Note: In mammals the thoracic cavity is completely separated
         from the abdominal by the diaphragm, but in birds and
         many reptiles the separation is incomplete, while in
         other reptiles, and in amphibians and fishes, there is
         no marked separation and no true thorax.

   2. (Zo["o]l.)
      (a) The middle region of the body of an insect, or that
          region which bears the legs and wings. It is composed
          of three united somites, each of which is composed of
          several distinct parts. See Illust. in Appendix. and
          Illust. of {Coleoptera}.
      (b) The second, or middle, region of the body of a
          crustacean, arachnid, or other articulate animal. In
          the case of decapod Crustacea, some writers include
          under the term thorax only the three segments bearing
          the maxillipeds; others include also the five segments
          bearing the legs. See Illust. in Appendix.

   3. (Antiq.) A breastplate, cuirass, or corselet; especially,
      the breastplate worn by the ancient Greeks.

Source : WordNet®

thorax
     n 1: the middle region of the body of an arthropod between the
          head and the abdomen
     2: the part of the human body between the neck and the
        diaphragm or the corresponding part in other vertebrates
        [syn: {chest}, {pectus}]
     3: part of an insect's body that bears the wings and legs
     [also: {thoraces} (pl)]