THe meaning of Prime I am familiar with, of course. It is understandable that various denizens of the other planes would find theirs to be centric.

My curiosity lies in the use of the word material.

If all planes are Material aside from the Astral Plane, then why not refer to them as Prime Material and Lesser Material?

... I am aware the question may seem a touch pedantic, I can assure you that is not my intention, but it's curious to me. Are there physical differences between the two worlds?