It's the site you're on, so in your case: lemm.ee.

Instance in this situation is usually the same as website.

From other instances, or if you view Lemmy from your browser while signed out, people's usernames are shown in full. If you share a link to this post with yourself and view it while signed out, you'll see your full username - SouthEndSunset@lemm.ee

The part after the @ is your instance, and just for completion, your username is the part before it, making you SouthEndSunset on the Lemm.ee instance :)

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