Reusing material from thesis proposal in final thesis I'm writing my final thesis and there's a lot of material I would like to reuse from my proposal -- introductions, motivations, etc. -- and it would be nice to just copy-paste (and update a bit) entire chapters from the proposal. 

But then I started to wonder what the technical status of a proposal really is. It's not archived anywhere, it's not "published" in any traditional sense. Is it a sort of first draft of the final thesis? Is it possible/legal/acceptable to reuse portions of it in the final thesis? Or is that plagiarism?
Your advisor should be able to provide you with the guidance. 

In general, it is not plagiarism, and you can reuse the material. As you pointed out the proposal is not published. It is a confidential document outlining what you intended to do and is to be shared only with your committee.