thorax (thôrʹăks´, thōrʹ-) noun. plural thoraxes or thoraces. 1. 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. 2. A part in other vertebrates that corresponds to the human thorax. 3. The second or middle region of the body of an arthropod, between the head and the abdomen, in insects bearing the true legs and wings, toraks.