Definition:Diameter of Quadrilateral
Let $ABCD$ be a quadrilateral:
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.
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.