soc180:glossary

Matrix (usually square) with zero elements except on main diagonal (See also: wikipedia)
An arrangement of a set of numbers into rows and columns like this:
$\begin{bmatrix}1 & 1 & 0 \\0 & 0 & 1 \\0 & 1 & 0 \end{bmatrix}$
Each element of the matrix is referred to by subscripts, the first representing the row and the second the column. Thus A_{23} refers to the element in the second row, third column. More generally, A_{ij} refers to the element in the i^{th} row and j^{th} column.