Not on the official list. Some things are supposed to be entered in a record kept by a government, a school, a company, or a website — cars, guns, businesses, voters, students, phone numbers, users of an app. If that entry was never made, the thing or the person is unregistered. "An unregistered vehicle." "Thousands of unregistered voters." "Unregistered users can read the articles but cannot comment." The word usually points at a problem. Unregistered often means illegal or at least outside the rules: an unregistered gun, an unregistered business, an unregistered clinic. But not always — an unregistered user on a website is doing nothing wrong, they simply have not made an account yet, and an unregistered guest is just someone who has not signed in. So read the situation before you assume blame. It also appears in software, where an unregistered copy of a program is one you have installed but not paid for or activated, and it keeps nagging you about it. That use is a little dated now that everything is a subscription. Note the shape of the word: it is un- plus a past participle, and it describes a state, not an action. If you want the action, use the verb: "I need to register my scooter."