Recently, a post from GeoCurrents went viral for its explanation of unconventional geopolitical status in the world, and in it, we find the best possible definition for the commonwealth there is:
Incongruous Commonwealth: Puerto Rico indicated. Puerto Rico, like the Northern Mariana Islands, occupies an incongruous geopolitical position. It is not fully part of the US, but neither is it a mere dependency (as are Guam, American Samoa, etc). Puerto Rica is instead officially classified as a “commonwealth.” Four U.S. states (Virginia, Kentucky, Pennsylvania, and Massachusetts) also have official “commonwealth” status. There is no connection between the two kinds of commonwealths. In fact, in neither instance does the term “commonwealth” have any real meaning at all.
Martin Lewis, GeoCurrents
Someone forward this to Senators Lisa Murkowski and Martin Heinrich.
Otherwise they can go with Ruben Berrios’s definition, which is just as good: