Diagonal means either a diagonal matrix (usually a square matrix) in which the entries outside the main diagonal are all zero. The diagonal entries themselves may or may not be zero. Or, the values of the diagonal matrix.
Since a lot of things in programming is based on matrices doing thing horizontal or vertical is often easier then doing it diagonal. Questions dealing with these difficulties should be tagged diagonal.
Diagonal matrices occur in many areas of linear algebra. Because of the simple description of the matrix operation and eigenvalues/eigenvectors, it is always desirable to represent a given matrix or linear map by a diagonal matrix.
You probably want to add more specific tags about the technology you are using. Areas where the tag ´diagonal´ becomes relevant often drawing and animation.
