A group of 9 diagonally connected cells. In Sudoku-X and other variants, the diagonals must also contain digits 1 through 9.
When the diagonals are used as an extra constraint, they are not considered as extra divisions, because they do not cover all cells in the grid. Also, both diagonals intersect at r5c5, making it the only cell in the grid that belongs to 5 houses.
The main diagonal runs from left-bottom to right-top. The anti-diagonal runs from left-top to right-bottom. Note: these terms are reversed in other sources - see the discussion page.