Main diagonal

In linear algebra, the main diagonal (sometimes principal diagonal, primary diagonal, leading diagonal, or major diagonal) of a matrix formula_1 is the collection of entries formula_2 where formula_3. All off-diagonal elements are zero in a diagonal matrix. The following three matrices have their main diagonals indicated by red 1's: 
Antidiagonal.
The antidiagonal (sometimes Harrison diagonal, secondary diagonal, trailing diagonal, minor diagonal, or bad diagonal) of a dimension formula_5 square matrix, formula_6, is the collection of entries formula_7 such that formula_8 for all formula_9. That is, it runs from the top right corner to the bottom left corner.