peritoneum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The serous membrane that lines the walls of the abdominal cavity and folds inward to enclose the viscera.

from The Century Dictionary.

• noun The membrane lining the abdominal cavity and investing its viscera.
• noun In brachiopods, an investment of the alimentary canal, by which the latter is suspended in the perivisceral cavity as by a mesentery.
• noun In entomology, the outer coat of the digestive tube of an insect.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) The smooth serous membrane which lines the cavity of the abdomen, or the whole body cavity when there is no diaphragm, and, turning back, surrounds the viscera, forming a closed, or nearly closed, sac.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy In mammals, the serous membrane lining the cavity of the abdomen and that is folded over the viscera.
• noun zoology In animals, the membrane lining the coelom cavity.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a transparent membrane that lines the abdominal cavity in mammals and covers most of the viscera