Source : Webster's Revised Unabridged Dictionary (1913)

Endocardium \En`do*car"di*um\, n. [NL., fr. Gr. 'e`ndon within +
   ? heart.] (Anat.)
   The membrane lining the cavities of the heart.

Source : WordNet®

endocardium
     n : the membrane that lines the cavities of the heart and forms
         part of the heart valves
     [also: {endocardia} (pl)]