Source : Webster's Revised Unabridged Dictionary (1913)

Mezzo-rilievo \Mez"zo-ri*lie"vo\, n. [It.]
   (a) A middle degree of relief in figures, between high and
       low relief.
   (b) Sculpture in this kind of relief. See under
       {Alto-rilievo}.

Source : WordNet®

mezzo-rilievo
     n : a sculptural relief between low relief and high relief [syn:
          {mezzo-relievo}, {half-relief}]