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.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.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.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.n. In ascidians (Tunicata), a special posterior portion of the body, situated behind the great pharyngeal cavity, and containing most of the alimentary canal.