abdomen

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The part of the body that lies between the thorax and the pelvis and encloses the stomach, intestines, liver, spleen, and pancreas. Also called belly.
2. n. The corresponding region in vertebrates other than mammals.
3. n. The posterior segment of the body in arthropods.

Century Dictionary and Cyclopedia

1. n. The belly; that part of the body of a mammal which lies between the thorax and the pelvis; the perivisceral cavity containing most of the digestive and some of the urogenital organs and associated structures. It is bounded above by the diaphragm, which separates it from the thoracic cavity; below by the brim of the pelvic cavity, with which it is continuous; behind by the vertebral column and the psoas and quadratus lumborum muscles; in front and laterally by several lower ribs, the iliac bones, and the abdominal muscles proper. The walls of the abdomen are lined with the serous membrane called peritoneum, and are externally invested with common integument. Its external surface is arbitrarily divided into certain definite regions, called abdominal regions (see abdominal). The principal contents of the abdomen, in man and other mammals, are the end of the esophagus, the stomach, the small and most of the large intestine, the liver, pancreas, and spleen, the kidneys, suprarenal capsules, ureters, bladder (in part), uterus (during pregnancy at least), and sometimes the testicles, with the associated nervous, vascular, and serous structures. The apertures in the abdominal walls are, usually, several through the diaphragm, for the passage of the esophagus, nerves, blood-vessels, and lymphatics; in the groin, for the passage of the femoral vessels and nerves and the spermatic cord, or the round ligament of the uterus; and at the navel, in the fetus, for the passage of the umbilical vessels.
2. n. In vertebrates below mammals, in which there is no diaphragm, and the abdomen consequently is not separated from the thorax, a region of the body corresponding to but not coincident with the human abdomen, and varying in extent according to the configuration of the body. Thus, the abdomen of a serpent is coextensive with the under side of the body from head to tail; and in descriptive ornithology “pectus is restricted to the swelling anterior part of the gastræum, which we call belly or abdomen as soon as it begins to straighten out and flatten.” Coues, N. A. Birds, p. 96.
3. n. In entomology, the hind body, the posterior one of the three parts of a perfect insect, united with the thorax by a slender connecting portion, and containing the greater part of the digestive apparatus. It is divided into a number of rings or segments, typically eleven (or ten, as in Hymenoptera and Lepidoptera), on the sides of which are small respiratory stigmata, or spiracles.
4. n. In Arthropoda other than insects, the corresponding hinder part of the body, however distinguished from the thorax, as the tail of a lobster or the apron of a crab.
5. n. In ascidians (Tunicata), a special posterior portion of the body, situated behind the great pharyngeal cavity, and containing most of the alimentary canal.

GNU Webster's 1913

1. n. (Anat.) 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.
2. n. (Zoöl.) The posterior section of the body, behind the thorax, in insects, crustaceans, and other Arthropoda.

WordNet 3.0

1. n. the region of the body of a vertebrate between the thorax and the pelvis
2. n. the cavity containing the major viscera; in mammals it is separated from the thorax by the diaphragm