Where does one draw the line to determine what is ethical to share and what isn't?
You can avoid plagiarism the same way you would in a research paper: by citing your sources! So long as you’re not just quoting the same sources over and over again, you can quote a short excerpt from a relevant source and cite it. It’s also worth noting that definitions of common terms, as well as equations, are not normally subject to copyright, because of the exact reason you mention: there are only so many legitimate ways to define a term or write an equation, which places a constraint on expressing them without using someone else’s definition.

As far as links go, there’s no reason you can’t cite what you want.