Definition:Diameter of Quadrilateral

Definition
Let $ABCD$ be a quadrilateral:


 * DiameterOfQuadrilateral.png

The diameters of $ABCD$ are the lines $AC$ and $BD$ joining their opposite vertices.

Also known as
The diameters of a quadrilateral are also commonly known as its diagonals.

Also see
Note the difference between this definition and that of the diameter of a general geometric figure.

In the latter, the diameter is specifically the longest dimension of the figure.