Think of arthropathy as just another word for arthritis or osteoarthritis. The facet joints are joints on either side of each level of the spine that alow for movement of the spine. Facet arthopathy is just arthritis of these joints and a term often used by people reading x-rays or MRIs. Facet arthrosis is also used meaning the same thing. Another general term for this type of arthritis is degenerative joint disease.

Facet is a noun.

The facet joints are located in the spinal canal. Bilateral facet arthropathy is arthritis in the facet joints of the back.

Let's consider this issue from another facet.

In anatomy, a facet is a classification of bones according to their bone surface features. A facet is a small, flat, articular surface.