- (anatomy) 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. [First attested in the early 17th century.]
"He was all bent over complaining of pains in the abdomen."
- (anatomy) 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. [First attested in the early 17th century.]
- (obsolete) The fat surrounding the belly. [Attested from the mid 16th century until the late 17th century.]
- (zoology, entomology) The posterior section of the body, behind the thorax, in insects, crustaceans, and other Arthropoda. [First attested in the late 18th century.]