per·i·car·di·um 
(pĕr′ĭ-kärdē-əm)
(pĕr′ĭ-kärdē-əm)n. pl. per·i·car·di·a (-dē-ə)
The membranous sac filled with serous fluid that encloses the heart and the roots of the aorta and other large blood vessels.
[New Latin, from Greek perikardion, from perikardios, around the heart : peri-, peri- + kardiā, heart; see kerd- in the Appendix of Indo-European roots.]
per′i·cardi·al (-dē-əl), per′i·cardi·ac′ (-dē-ăk′) adj.