Noun
the diagonal of a square matrix running from the upper left entry to the lower right entry
Source: WordNetThis follows immediately from the fact that transposing a square matrix does not affect elements along the main diagonal. Source: Internet
This implies that the (i, j)th entry of the table equals the (j, i)th entry, thus the table is symmetric about the main diagonal. Source: Internet
This is the "main diagonal" going through the origin. Source: Internet