Diagonal matrix

A diagonal matrix is a square matrix all of whose nonzero entries are along the main diagonal. Examples include the identity matrix, the zero matrix, and the matrix: $\left({\begin{matrix}1&0&0\\0&2&0\\0&0&3\end{matrix}}\right)$

The eigenvalues of a diagonal are the non-zero elements.

This is a special case of a Triangular matrix.