Diagonal matrix

A square matrix in which all entries — with the possible exception of the elements on the main diagonal — are zero.

Comments

I.e. an $(n\times n)$ diagonal matrix over a field $K$ has the form

$$\begin{pmatrix}a_1&0&\ldots&0\\0&a_2&\ldots&0\\\ldots&\ldots&\ldots&\ldots\\0&\ldots&\ldots&a_n\end{pmatrix},$$

where the $a_i$ are elements of $K$.