I don't have a suitable definition for Bitcoin or alt-coin, although I'm happy to make observations on what other people think.
IMO it would be useful to distinguish blockchain forks that happen intentionally to make an alternative system, and maybe that's deserving of a new word other than altcoin, particularly when it's an attempt at an usurpation as it happened with Feathercoin or what BitcoinXT would aim to achieve.
Maybe altfork? However nobody says that.
Frankly I think altcoin is a neutral term and sometimes, albeit not often, they are perfectly legitimate and worthy of praise (as opposed to the XT hackjob).
The other question is whether something can be simultaneously "Bitcoin" and "an altcoin" given this definition of altcoin in the wiki, and whether XT is that.
I can agree with defining Bitcoin XT as an "altfork", this does sufficient justice to separate altcoins from chain forks which I do think are separate phenomena.
Chain forks or "altforks" are indeed sometimes justified in order to avoid the danger of centralization of power that can develop within a Core development team. However if Bitcoin Core forks for any reason at all then I suppose Bitcoin Core should also be considered as being an "altfork", which you might even already be able to argue, based on previous forks in Bitcoins history. I have already written extensively on why I think such "altforks" are justified. Read the article that I wrote on the subject if you are more interested in my reasoning.