Is it plagiarism and/or copyright infringement if research started from one person but another diverged onto a different path?
Honestly, there's too less info provided here for strangers on the internet to guess and come to a conclusion. I'd recommend talking to your research guides to look into the content and sort out the matter. After-all, it's a few definitions, and you have branched off to create fresh research on your own anyway, so your research isn't in danger. "Pick and choose your battles". Some battles may not be worth the time and energy.
   Think about it this way: If you lived in a jungle and heard nothing about Newtons laws but you came up with those laws and definitions on your own and came to the city and tried publishing a book and then Newton accused you of plagiarism, what would be the right course of action? Unless you have hard evidence that you had not heard of Newtons laws earlier, nobody would believe you.