Basically how reddit would call any given reddit sub-community “sub reddit” or just “sub” for short. I know internally lemmy just calls them ‘community’ but in a regular conversation ‘community’ might be interpreted as more broad or too general.
Thinking about it my mind would pretty much automatically go to ‘sub lemmy’, but then I felt like just shortening it to ‘blemmy’ has a great ring to it, while still being distinct. So if there isn’t already an established name, I’ll go ahead and propose ‘blemmy’.
I’d say that “subreddit” is a brand name for “forum hosted on the Reddit service”. I don’t just mean in the legal sense — as you say, it tells you what service to find the forum on.
There isn’t a brand name for forums hosted on Lemmy; the local term being the generic “community” kind of ensures that. The compatibility between Lemmy and Kbin also means that I’m noticing the use of “Lemmy community or Kbin magazine” as an awkward generic phrase.
“Fediverse forum” or “ActivityPub forum” use what are likely even less-familiar service names.
So, I don’t know there’s a good answer yet!