In a magic cube, the four space diagonals are the lines that go from a corner of the cube, through the center of the cube , to the opposite corner. These lines are customarily called triagonals.

For the cube to be considered magic, these four lines must sum correctly.

The word triagonal is derived from the fact that as you travel down the line, three coordinates change. The equivalent in a magic square is diagonal, because two coordinates change. In a tesseract it is quadragonal because 4 coordinates change, etc.