The Collaborative International Dictionary of English v.0.48:

Epicardium \Ep`i*car"di*um\, n. [NL., fr. Gr. 'epi` upon +
   kardi`a heart.] (Anat.)
   That part of the pericardium which forms the outer surface of
   the heart; the cardiac pericardium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

epicardium
    n 1: the innermost of the two layers of the pericardium [syn:
         epicardium, visceral pericardium]