Reddit has subreddits. What does Lemmy go with?
My personal vote is for lemmings!
Edit: I am personally leaning towards Sublemmy now. It retains the context of being a forum under the general sphere of Lemmy and the connection to Reddit lets people know immediately what Lemmy is about. Thanks to @BurningnnTree@lemmy.one for the comment!
Is this a joke question? You used the term yourself: “Communities”.
On kbin, they are called “magazines”.