Abdomen: The belly, that part of the body that contains all of the structures between the chest and the pelvis. The abdomen is separated anatomically from the chest by the diaphragm, the powerful muscle spanning the body cavity below the lungs.
The abdomen includes a host of organs including the stomach, small intestine, colon, rectum, liver, spleen, pancreas, kidneys, appendix, gallbladder, and bladder.
The word "abdomen" has a curious story behind it. It comes from the Latin "abdodere", to hide. The idea was that whatever was eaten was hidden in the abdomen.