"x" in botanical nomenclature refers to a hybrid or a "crossing" (either natural or contrived) of two different species. Such a crossing or the resultant hybrid may or may not be intermediate between the two parent plants.

In many cases, the 'x' is dropped from common usage - eg. Thuja 'Green Giant', which is officially Thuja plicata x Thuja standishii - the names just become too unwieldy for popular usage. One can also have complex hybrids involving multiple species. The 'x' is typically dropped from these hybrids and they are usually referred to by their generic name or assigned a new species name (Helleborus x hybridus or H. x ballardiae).

One can also have intergeneric hybrids or a cross between two plants from different genera but these are less common.

OK, thanks! The x names that I was wondering about are thus x incomplete names (incomplete = without counterpart)