I am writing a paper with a co-author. I need to explain some terms in the preliminaries section. I am copying the definitions from my co-authors previous paper. Afterwards, I want to include the paper into my thesis. If I use the same definitions word for word in my thesis, would that be considered plagiarism?
Your question sounds like you're asking about a math paper. Definitions in mathematics are a bit of a strange corner case for plagiarism issues because they're not supposed to be "in your own words." My understanding of normal behavior is that you would never put a definition in quotes (unless you're writing about history), that you can nonetheless reuse definitions verbatim, but that you need to either cite the original source or say that they are standard definitions. A typical way to do this would be by saying "We recall some key definitions from X."
- If you are going to use their definitions verbatim, then use quotes, or otherwise make it very clear that the words are not your own.
- In any case, cite the sources.
If you use other peoples' words in your thesis, without making it very clear that they are other peoples' words, then yes, you would be committing plagiarism.
Granted, this case would not be extreme, compared with say, including a chapter from someone else's thesis in your own, but it does meet the definition. Given that many universities have adopted draconian anti-plagiarism policies in recent years, you should be very careful to avoid even minor infractions.