What is CANON?

IMO the term canon, when applied to a collection of fictional entertainment is thus... [b]Any installments created in the same format as the original installment is canon, everything else is extended universe or fan fiction unless expressively and officially stated otherwise.[/b] For example... - Alien, Aliens, Alien 3 and Alien Resurrection are canon. - Alien comics, books, graphic novels, toys and video games are not canon. - Batman comics such as Year One are canon. - Batman Begins Trilogy, Batman cartoons, and the 70's TV series are not canon. - Odd Thomas, Forever Odd, Brother Odd, Odd Hours and the forthcoming Odd Apocalypse are canon. - The forthcoming Odd Thomas movie is not canon. Of course there are exceptions to the rule, for example the Gears of War novels and comics are considered canon to the events in the games. Others have the view that what they like, or don't like determines what is canon, from their perspective - for example some fans refusing to accept Alien 3 and Resurrection as canon. And furthermore, someone stated in a thread yesterday (I am sure they will make themselves known) that they viewed a difference between "Franchise" and "Canon" - stating that all of the 4 Alien films are of the same franchise but that only the original is canon. The basis, as I understand it, for this argument was two fold 1. In Alien the universe was created, whereas Aliens, 3 and Resurrection where elaborate fan fiction by other directors that had deviated away from the source material. 2. The term "canon" does not apply to the Alien franchise because they have not been imagined or governed by guiding figure. Such as Gene Roddenberry overseeing Star Trek TNG, for example. These arguments stem from actual definitions, definitions that are not fit for purpose in todays entertainment industry. Going back to the Batman examples, the newer Batman stories were not envisioned by Bob Kane, but are considered canon by DC, the writers/artists and the fans. Therefore how can this argument be used in relation to the Alien franchise, especially when you take into account the fact the Alien franchise has been overseen by Brandywine Productions, who also oversaw the AVP's. Of course there are other views regading what determines something being canon, and the arguments go back and forth, usually degrading into the realm of semantics and opinions. For a while I have stated that IMO the AVP's are canon, some agree, some don't. But IMO all of the Aliens are also canon, some agree, but again some don't. Therefore how can we determine whether something is or isn't canon, when we can't even agree on what canon is.