A skew diagonal is a diagonal of a square matrix which is traversed in the "northeast" direction. "The" skew diagonal
(or "secondary diagonal") of an 
square matrix is the skew diagonal from 
to 
.
Skew Diagonal
See also
DiagonalExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Skew Diagonal." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/SkewDiagonal.html