Definition

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.

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.