The Collaborative International Dictionary of English v.0.48:

Pericardium \Per`i*car"di*um\, n. [NL., fr. Gr. perika`rdion,
   fr. perika`rdios about or near the heart; peri` about +
   kardi`a heart.] (Anat.)
   The double baglike fold of serous membrane which incloses the
   heart.
   [1913 Webster]

   Note: The inner layer is closely adherent to the outer
         surface of the heart, and is called the cardiac
         pericardium. The outer layer loosely incloses the heart
         and the adherent inner layer, and is called the
         parietal pericardium. At the base of the heart the two
         layers are continuous, and form a narrow closed cavity
         filled with fluid, in which the pulsations of the heart
         cause little friction.
         [1913 Webster]

WordNet (r) 3.0 (2006):

pericardium
    n 1: a serous membrane with two layers that surrounds the heart