The Collaborative International Dictionary of English v.0.48:

Isocephalism \I`so*ceph"a*lism\, n. [From Gr. ? like-headed. See
   Iso-, and Cephalon.] (Art)
   A peculiarity in the design of bas-relief by which the heads
   of human figures are kept at the same height from the ground,
   whether the personages are seated, standing, or mounted on
   horseback; -- called also isokephaleia.
   [1913 Webster]