noun
also peri·to·nae·um ˌperətəˈnēəm, -rət ə nˈē-, -rəd.əˈnē-
( plural peritoneums or perito·nea also perito·naea -ēə)
Etymology: Late Latin, from Greek peritonaion, neuter of peritonaios, stretched across, from peritonos stretched over, from peri- + -tonos (from teinein to stretch) — more at thin
1. : the smooth transparent serous membrane that lines the cavity of the abdomen of a mammal, is reflected inward over the abdominal and pelvic viscera, and consists of (1) an outer layer closely adherent to the walls of the abdomen except in some places along the back where it extends forward to form (2) an inner layer that folds to invest the viscera — called also (1) parietal peritoneum, (2) visceral peritoneum ; compare mesentery
2. : pleuroperitoneum
3. : the membranous lining of the body cavity of certain invertebrates