Diagonal is a line that connects one corner with the corner furthest away. An example of diagonal is a line going from the bottom left corner of a square to the top right corner.
The word "diagonal" originates from the ancient Greek 'diagnosis,' meaning "from angle to angle." In matrix algebra, a diagonal makes a set of entries that are increasing from one corner to the farthest corner. There are also other, non-mathematical uses of diagonal.
A diagonal line creates triangular shapes as it interacts with the frame. Diagonals are most attractive when they interact with horizontal lines and an opposite diagonal, which produces complex sets of triangles. Long diagonals may create big triangles that function as arrows.