Abdomen n. : The belly, or that part of the body between the thorax and the pelvis. Also, the cavity of the belly, which is lined by the peritoneum, and contains the stomach, bowels, and other viscera. In man, often restricted to the part between the diaphragm and the commencement of the pelvis, the remainder being called the pelvic cavity.

Abdomen n. : The posterior section of the body, behind the thorax, in insects, crustaceans, and other Arthropoda.

Example:
What part of your Abdomen begins to grow first during pregnancy?

Taxonomy
human abdomen, segments, abdominal segments, vertebrates, free encyclopedia, rapper, abdominal cavity, digestive tract, appendages, Arthropod, body, Abdominal organs, thorax, cavity, organs, The American Heritage Dictionary of the English Language, aorta, appendix.

Defination from the web:
abdomen is a large cavity enclosed by the abdominal muscles, ventraly and lateraly, and by the vertebral column dorsally. ... In the worker ant, the abdomen consists of the propodeum fused to the thorax and the metasoma, itself divided into the narrow petiole and bulbous ...