Definition:Diameter of Parallelogram
Let $ABCD$ be a parallelogram:
The diameters of $ABCD$ are the lines $AC$ and $BD$ joining their opposite vertices.
Also known as
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.
Also see Diameter of Quadrilateral, of which this is a special case.