What counts as "canon"?

[Ashbranch]

Originally Posted by Lalwendë

Yeah, you could even start asking which edition of each text is the only canonical one

Does a first or the latest edition of Lord of the Rings have greater authority?

I suppose Tolkien's most recent version of a thing is the most authoritative - hence Strider and not Trotter, say. But do we then extend this to "corrections" made posthumously? Recent editions of LotR have "corrected" the info about the kings of Numenor so that it conforms to that found in Unfinished Tales, for example.