From Marco Palmieri in Voyages of Imagination:
From Paula Block, Director of Licensed Publishing for CBS Consumer Products and the studio's primary editorial contact in matters relating to Star Trek fiction, from the same book:
But for the rest of us, I think Paula's definition works.
And, of course, alternate universes are helpful too.
In the context of Star Trek, canon refers to the body of information the show and film writers adhere to when crafting new stories. That's all it means. Unfortunately, the word is frequently misunderstood and misapplied as a value judgment: 'Such-and-such a story is canon to me.' Or, 'Such-and-such a story isn't part of my personal canon.' This is the wrong way to use the word. It's not subjective.
It's also frequently and incorrectly used in place of the word 'continuity' or 'consistency.' These terms are not interchangeable.
From Paula Block, Director of Licensed Publishing for CBS Consumer Products and the studio's primary editorial contact in matters relating to Star Trek fiction, from the same book:
I think the Great Bird gets a pass.Canon is what is seen on TV and movie screens. End of story....
Which is not to say that there haven't been times when canon has contradicted itslef; those darn producers and scriptwriters don't always keep track of/remember/care about what's come before. So things can get confusing....
Gene [Roddenberry] had a habit of 'de-canonizing' (if there is such a thing) things when he wasn't happy with them. He didn't like the way that much of the animated series turned out, so he proclaimed that it was NOT CANON. He also didn't like a lot of the movies. So he didn't consider all of them canon either... Gene's view of canon was, I think, pretty fluid. He thought of TNG as canon wherever there was confict between [TNG and TOS]. He admitted it was revisionist thinking, but so be it.... That's kind of like God telling you the stuff in that old bible...well, he [was] just not that into it anymore.
You can see why canon is such a difficult concept.... But I always fall back on the first and original rule-- what you see on the big and small screens is canon. When you stray too far away from that basic rule, you venture into the land of arbitrary decisions and personal biases -- and life just isn't long enough to spend a lot of time dwelling on non-issues like that.

And, of course, alternate universes are helpful too.