1. A user of Ubuntu Linux. Also Ubuntite.
2. An Ubuntard.
3. An Ubuntero is a person in the Ubuntu community who has committed to observe the Ubuntu Code of Conduct. Ubunteros are self nominated, and you can become one by signing the code of conduct online (launchpad.net/codeofconduct).
4. A member of the social network for Ubuntu users, developers, and fans at ubuntero.org.
There is more to Linux than Ubuntu, but Ubunteros are often not aware of that.