Define a diagonal matrix.

A square matrix is said to be a diagonal matrix, if all its non-diagonal elements are zero.

An Example of diagonal matrix can be given as:
if $\left[ \begin{matrix}

2 & \phantom{-} 0 & \phantom{-} 0 \\

0 & \phantom{-} 1 & \phantom{-} 0 \\

0 & \phantom{-} 0 & \phantom{-} 5 \\

\end{matrix} \right]$