I answered a question from a new user. I was checking over my recent answers when I noticed that next to the new user it says "TheirUsername (Unregistered)". Does this mean they deleted their account or deleted their account after asking the question? (I hope I didn't scare them off with my answer, I didn't use any expletives!).
According to this answer on the main meta site, it's this:
They haven't registered themselves on the site. They are logged in with a long living cookie tied to a specific PC/webbrowser. They won't be able to login using the same account on other PCs/webbrowsers.