* Diagonal line: A line that connects two non-adjacent vertices of a polygon or polyhedron.
* Diagonal matrix: A square matrix where all the elements off the main diagonal are zero.
* Diagonalization: A mathematical process of transforming a matrix into a diagonal form.
The specific meaning of "diag" depends on the context.
Could you provide more context about where you encountered this term? That would help me give you a more accurate answer!