per·i·car·di·um / ˌperiˈkärdēəm/ • n. (pl. -car·di·a / -ˈkärdēə/ ) Anat. the membrane enclosing the heart, consisting of an outer fibrous layer and an inner double layer of serous membrane.DERIVATIVES: per·i·car·di·al / -ˈkärdēəl/ adj.

pericardium A conical bag which holds the heart. There is an outer fibrous covering, and within this a more delicate membrane: a closed sac, into which the heart is invaginated from behind, so that the two layers of membrane are in contact with each other. There is thus a potential space between the layers, which may be expanded with fluid in the condition of pericarditis, or with blood in penetrating injuries; since there is no exit, accumulated fluid interferes with the pump action, and needs to be aspirated.

Stuart Judge

pericardium (pericardial membrane) The membrane that encloses the pericardial cavity, containing the vertebrate heart. The pericardium holds the heart in position while allowing it to relax and contract. It consists of two main parts: a tough outer fibrous layer (fibrous pericardium) and the more delicate serous pericardium, which consists of a double layer of serous membrane, the inner layer being in close contact with the heart.

pericardium (pe-ri-kar-diŭm) n. the membrane surrounding the heart. fibrous p. the outer portion of the pericardium, which completely encloses the heart and is attached to the large blood vessels emerging from the heart. serous p. the internal portion of the pericardium: a closed sac of serous membrane containing a very small amount of fluid, which prevents friction between the two surfaces as the heart beats.
—pericardial adj.

pericardium XVI. — modL. — Gr. perikárdion, f. PERI- + kardíā HEART.
Hence pericardiac XIX, pericardial XVII, pericarditis XVIII.