A perfect magic cube is a Magic Cube for which the cross-section diagonals, as well as the space diagonals, sum to the Magic Constant.
See also Magic Cube, Semiperfect Magic Cube
References
Gardner, M. ``Magic Squares and Cubes.'' Ch. 17 in Time Travel and Other Mathematical Bewilderments.
New York: W. H. Freeman, pp. 213-225, 1988.