thorax

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The part of the human body between the neck and the diaphragm, partially encased by the ribs and containing the heart and lungs; the chest.
• noun A part in other vertebrates that corresponds to the human thorax.
• noun The second or middle region of the body of certain arthropods, including the insects and most crustaceans, lying between the head and the abdomen.

from The Century Dictionary.

• noun In anatomy and zoology, a part of the trunk between the head or neck and the abdomen or tail, in any way distinguished, as by containing the heart and lungs, by being inclosed with large ribs, or by bearing certain limbs not borne elsewhere.
• noun In entomology, that part of the body which is situated between the head and the abdomen, and in adult insects alone bears the wings and legs, when there are any. ;
• noun In Crustacea and Arachnida, a part of the body in advance of and in any way distinguished from the abdomen or tail, but usually blended with the head to form a cephalothorax.
• noun A breastplate, cuirass, or corselet; more especially, the cuirass or corselet worn by the ancient Greek warriors, corresponding to the lorica of the Romans.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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 vertebræ, the ribs, and the sternum, and which the heart and lungs are situated; the chest.
• noun 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.
• noun 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.
• noun (Antiq.) A breastplate, cuirass, or corselet; especially, the breastplate worn by the ancient Greeks.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the middle of three distinct divisions in an insect, crustacean or arachnid body
• noun the region of the mammalian body between the neck and abdomen as well as the cavity containing the heart and lungs

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the part of the human torso between the neck and the diaphragm or the corresponding part in other vertebrates
• noun part of an insect's body that bears the wings and legs
• noun the middle region of the body of an arthropod between the head and the abdomen