The meaning of Diagonal

adjective

(of a straight line) joining two opposite corners of a square, rectangle, or other straight-sided shape.

Usage examples:

Draw two diagonal lines dividing the square and the rectangle into triangles.

noun

A straight line joining two opposite corners of a square, rectangle, or other straight-sided shape.

Usage examples:

Do this at a diagonal, from one corner of the square to the opposite corner.

adjective

(of a line) straight and sloping, so that it is neither horizontal nor vertical

Usage examples:

A diagonal line in a circle is often used as a symbol to show that certain behavior is forbidden.