The Greek word kosmos Strong's #2889), usually translated "world," basically means, "order, regular disposition and arrangement." It can mean "adorned" (1 Pet 3:3), the earth as the home of humans, and in a pejorative sense, human society outside of God's Kingdom. Sometimes the context indicates a narrower meaning.
That last meaning is the basis of using the description "worldly," usually meant in a derogative sense by JWs (and probably others too). To be honest, I think the Society has said something about not using the term in reference to individuals. Off hand I can't find that counsel.
Titus 2:12 uses it with regard to "desires." And it can be found numerous times in the Publications Index.
I find the use of it against individuals as demeaning, but at the same time I know I've been guilty of using it in the past. I think as I've woken up some I've become more cognizant and sensitive about demeaning terms. I think Titus 3:1-3 would argue against using terms like that to belittle others. No doubt there are other similar verses.
"Christendom," to the Society means appostate Christianity. To the rest of the world it means areas where the Christian religion holds sway. The world would probably lump the WT along with "Christendom." "Christendom" would lump JWs outside of Christianity. And JWs would say only they are truly Christian. Its curious, isn't it?
Whenever the Society quotes from a non-WT reference work, if it supports a WT idea, then, the reference work is from a "Bible scholar." If it opposes a WT idea it is from "Christendom's clergy." Such are the techniques of propaganda.
Hope this helps