Define a scalar matrix.

A diagonal matrix is said to be a scalar matrix if its diagonal elements are equal.

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

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

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

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

\end{matrix} \right]$