They are called "lemmy communities" or "communities"

"Sublemmies" is cringe and is some weird portmanteau with subreddit which is stupid because I don't know why people are still attached the idea of subreddits since this isn't reddit and "lemmings" which some users like to call lemmy users but I don't agree with that either and "lemmy users" sounds like a better term to use

So many people here are trying to emulate reddit when this isn't reddit, yes some features from reddit would be nice like a group collection of subreddits like multireddits did and post flairs but this still isn't reddit