Or the definition of the third quadrant of the vernal galaxy?
The classification scheme is a mess.
We have gas giants under two classifications: an alphabet soup and a number soup.
We have classifications referring to size, to atmosphere, and to general characteristics of an astronomical object.
I would say Gene Roddenberry was ignorant of astronomical subjects as much as the NBC execs. In 1965, Mariner 4 had reached Mars and was transmitting data back to NASA that revealed that Mars was not Earth-like.
Though it is in its infancy, I think we are seeing how planets might be classified in the future. They are being classified on size, mass, atmosphere, and land-sea ratios. I might have missed a few. And, I think that there will be one body regulating the classification scheme so that it doesn't become a confusing mess.