Actually I would like to ask two questions and the answers may not be the same.

- When writing a paper and if one need to introduce a definition, is it alright to just copy-paste from wikipedia/a textbook/a paper, or should one at least rephrase it a little bit?
- If I need to copy a theorem from an old paper, is it better just to copy exactly the theorem as it is, or should I rephrase it to make it in harmony with my notation/modern terminology, etc.?

**Answer**

Well, there is no law against using copy-paste from any document unless it has a copyright (in which case even rephrasing is a form of plagiarism). If you think the original is too unclear, you could rewrite using your own terms, as long as you don’t change the meaning (even slightly). If it is clear enough, it’s best to copy it as it is.

**Attribution***Source : Link , Question Author : Zuriel , Answer Author : ghosts_in_the_code*