It is a simple enough question, but one that came up a few times in recent discussions over scifi writing. In scifi settings, many times a single government body has control over a planet, system, or dozens of star systems. Somehow, 'nations' no longer feels like an appropriate word to refer to these entities in general.
Right, but what I'm looking for is something general like nation. Maybe it simply does not exist because we have not culturally ever had a need for it. But 'those nations on the eastern quadrant' doesn't really sound right in my brain, and things like 'realms' has certain connotations that may not fit anywhere but a 'Dune' environment either.
Yeah I feel you with realms. That word evokes fantasy for me, not sci-fi. In Stars in my Pocket like Grains of Sand, (a perennial favorite of mine) Delany handled it by giving the "nations" names and simply calling them that (the Family & the Sygn) and these also covered huge stretches of space.
Babylon 5 referred to their different race's territories as "sphere of influence". But then I used "territories" in that sentence and it made perfect sense, so I would use that.
I'm not sure what you're looking for if Wreybies' examples didn't work for you? According to that definition, a nation is defined by its boundaries. If those boundaries are constantly spreading, like in space colonization, it can hardly be the unifying factor. I think it's likely that words representing union by other means would be more accurate: Union Domain Company Empire, like Wrebies said. (The British Empire comes to mind as a historical example.)
Your best bet would be to use, "Dominion." Not only does it sound good, but I think it is what you are looking for. "those dominions on the eastern quadrant...." courtesy of dictionary.com: do·min·ion [duh-min-yuhn] control; domination. 3. a territory, usually of considerable size, in which a single rulership holds sway.