Abdomen meaning : The fat surrounding the belly. The belly, or that part of the body between the thorax and the pelvis, not including the back; or in some lower vertebrates, the portion between the cardiac and caudal regions. The cavity of the belly, which is lined by the peritoneum, and contains the viscera; often restricted in humans to the part between the diaphragm and the commencement of the pelvis, the remainder being called the pelvic cavity. The posterior section of the body, behind the thorax, in insects, crustaceans, and other Arthropoda.