Definition:Diameter of Parallelogram

Let $$ABCD$$ be a parallelogram:


 * DiameterOfParallelogram.png

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

Compare with
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.