Honestly, I'm of the mind that canon is a poor choice of words for dealing with issues of "what is and isn't true in Star Trek/whatever franchise." Canon is a reference to the issue of the Holy Catholic Church's view of what is and isn't true about the nature of the universe and God. So, it's a spiritual co-opting. However, the thing is that it inherently loses some of its power when dealing with works of fiction.
Quite right. It's a metaphor to begin with, so it shouldn't be taken too seriously.
Also, defining it in terms of "true" seems misguided when talking about fiction. You can't try to judge fiction by the same standards as reality, because it's the nature of fiction to be mutable. Human storytelling has always been a changeable thing. Before widespread literacy and printing, when stories were mostly oral, they changed every time they were told -- just like human memory changes slightly every time it's recalled. What exists in our minds is intrinsically more fluid than what exists in objective reality. So you can't really talk about "truth" in a story (emotional truth, yes, but not objective, factual truth). There's just whatever interpretation the storyteller is working from at a given moment. Ideally, storytellers remember past stories well enough to create a convincing illusion of consistency, but there will always be small-scale changes, whether due to error or due to deliberate correction and refinement of past imperfections.
No, Star Wars: The Novelverse is not canon to the television show.
Lately I'm seeing a lot of people say
Star Wars when they mean
Star Trek. Gee, I wonder why people are so preoccupied with
Star Wars lately.
and Human Nature being a actual adaptation--which, ironically, caused a minor fit in the fandom because if it was showing up on the show then it couldn't have happened in the books. An odd but actual complaint.
To me it seems self-evident that the TV version overwrites the novel version, but I don't see that as a matter for moral outrage, just a matter of classification to be worked out. It would be a tricky one, though, because the novel
Human Nature was part of a fairly serialized novel line whose books made constant reference to each other (indeed, that's why I lost interest in it, because I couldn't afford to collect every volume and started to feel lost when I missed some installments). So it would be hard to remove just one book without screwing up everything after it.
I wonder if people get angry about continuity changes affecting tie-ins because they believe that they're dependent on studios to declare what is or isn't "real" and thus feel they have no control over it. I've always seen it as my own personal decision whether or not to count a tie-in as part of the continuity. So if new canon (or a later tie-in I like better) comes along and requires removing a tie-in (or a whole series of them) from my personal continuity, I may well be annoyed, but I don't feel cheated or betrayed, because it was my own choice to count them in the first place. I simply have to make some new choices in response to the changes.
Do you have a better term that's equally concise? (And don't say "in continuity", that's twice as long to write and twice as long to say; who has time for that?

)
Part of the problem is that people dwell too much on the label itself. Labels do more to get in the way of understanding than to improve it. Fictional continuity is a complicated issue, and the only way to get a real understanding is to be open to that complexity and nuance and the many factors that go into it. Trying to simplify it to a single five-letter shibboleth that somehow explains everything is a pointless exercise. "Canon" can be a convenient shorthand to use as a term of criticism, but only if you understand its limitations and the larger context in which it resides.
I recall as a child in the 80s my brother mentioned that Paramount had a policy of not referencing other books and deliberately going out of their way NOT to keep things consistent between volumes. Then again, Star Trek came back to the screen in 1987 and was constantly throwing out new information as the writers came up with it about the Klingons, Romulans, and so on so the novels were always destined to be non-canon in the eyes of fans.
That policy didn't come along until the '90s, though. In the '80s, while Pocket had no uniform continuity, there was an increasing trend for different novels to reference each other, and a
loose novel continuity did emerge. The crackdown didn't happen until
after TNG was around, when Roddenberry and Richard Arnold started to feel the tie-ins were creating too much confusion over what was "real" in the Trek universe. This led to the shutdown of DC's first TOS series in 1988 and its revival the following year with a more continuity-light format (in theory, though Peter David and Richard Arnold clashed extensively over the matter until David left the book after a year and a half), and to the end of the loose Pocket continuity by c. 1990. The '90s Pocket novels stuck to a pretty strict no-continuity policy for most of the decade, even well after Arnold was no longer overseeing the tie-ins. That started to erode with
New Frontier and eventually with the emergence of the modern novel continuity by 2000.
Sure, because reading the books as they were published, it was obvious that they were not always consistent.
Yes, absolutely. They were often hugely inconsistent with each other, not just in continuity details but in their whole characterization of the universe. So little about the Trek universe had really been defined by TOS that there was plenty of room to fill in the background, and different authors brought their own visions and voices to it, extrapolating its technology and culture and history in different ways. That was part of what was interesting about those early novels, the fact that you never knew what you were going to get. It was like an art class where a bunch of students painting the same subject all interpret it in different styles -- one might be realist, one impressionist, one cubist, one pop art, etc. In the Arnold era, it was more like a class where everyone was required to be strictly realist. These days it's sort of in between -- we're allowed to be fairly individualistic in our expression, but since so much more of the universe is defined and codified, there are fewer areas that are open to interpretation.
Richard Arnold began speaking on behalf of the Star Trek Office from late 1986, and that tends to be fandom's first use of the term "canon".
Well, as
TheAlmanac showed above, some fans were using the term before then, in '84-'85. But as I said, it was probably limited more to use by more scholarly sorts who were aware of the jargon of media/literary criticism. What Arnold did was to popularize it more widely among laypeople -- people who weren't versed in the underlying ideas and methods of analysis that went with the term and were thus prone to misinterpret and mythologize it, especially since their main referent for it was Arnold's own judgmental and fundamentalist approach to it.