Communities. It is an established name.

• The documentation uses it: https://join-lemmy.org/docs/en/users/01-getting-started.html
• The browser interface uses it (see top left, see in dropdown menus)
• The unchangeable URL structure uses it (we are in /c/linux@lemmy.ml)

Lemmy is difficult enough to learn for newcomers. Please don’t make it harder for them to join. Don’t force them to learn two terms where one suffices.