Source : Webster's Revised Unabridged Dictionary (1913)

Myocardium \My`o*car"di*um\, n. [NL., fr. Gr. ?, ?, muscle + ?
   heart.] (Anat.)
   The main substance of the muscular wall of the heart inclosed
   between the epicardium and endocardium.

Source : WordNet®

myocardium
     n : the middle muscular layer of the heart wall
     [also: {myocardia} (pl)]