TITLE: What is the equivalent of a diagonal in a non-square matrix or array?
QUESTION [14 upvotes]: I have a non-square matrix $M$, that looks something like this:
$M=\left[
\begin{array}
& a & b & c \\
d & e & f \\
g & h & i \\
j & k & l \\
\end{array}\right]$
I would like to refer the cells $M_{ij}$ where $i=j$, like $\{a,e,i\}$ in this case. If this was a square matrix, that would be the diagonal. Is that still the right word, or is there something more fitting that avoids confusion?

REPLY [16 votes]: Yes, those entries: $m_{ij}$ in $M$, with $i = j$, constitute what can technically be called the "main diagonal" of the rectangular matrix, though the diagonal of such a matrix is not necessarily as "useful" as it is in a square matrix. See, e.g., the Main Diagonal entry in Wikipedia.