per·i·to·ne·um also per·i·to·nae·um 
(pĕr′ĭ-tn-ēəm)
(pĕr′ĭ-tn-ēəm)n. pl. per·i·to·ne·a also per·i·to·nae·a (-tn-ēə)
The serous membrane that lines the walls of the abdominal cavity and folds inward to enclose the viscera.
[Middle English, from Late Latin peritonaeum, from Greek peritonaion, from peritonaios, stretched across, from peritonos, stretched around : peri-, peri- + teinein, to stretch; see ten- in the Appendix of Indo-European roots.]
per′i·to·neal adj.
per′i·to·neal·ly adv.