This is a phrase that gets thrown around a lot. I'm pretty sure I didn't come up with it myself [I think I got it from someone else, but can't remember who] but various people have cited me as the one who coined the phrase here at ZU
...but it seems everyone has a different meaning for it
So, in order to avoid confusion, explain what you mean when you deem something to be "semi-canon". I'll go first.
If there's a game or resource that is clearly not part of the primary evidence category, but still considered in my mind to be an appropriate source of proof, I deem it "semi-canon". It's not indisputably canon, such as the 14 main Zelda titles, but I can find nothing in it to be contradicted by the games. This term to me is virtually synonymous with "secondary evidence".
Because it is not indisputably canon, nor to be considered a primary source of information, I do not base
theories on semi-canon material. Any theory that uses semi-canon material must have its base in a primary source of canon information, and the semi-canon elements should only be used as supporting evidence.
Another distinction between semi-canon and canon is that I don't expect others to necessarily agree with me on its relevance. While I'll generally ask someone to explain why they don't consider it valid, I don't hold it as undeniable.
An example of this distinction would be that some people claim that the linked ending of the Oracles, and games like Four Swords and Four Swords Adventures are not canon. I believe these things to be fully canon, and see no reason why their validity should be disputed, and I won't hesitate to make that known to the person. However, in the case of a non-primary source of information, such as the spinoff "Freshly Picked: Tingle's Rosey Rupeeland", I don't expect others to believe its canonicity.
That's my explanation, whats yours?