These are all just terms for slightly different perspectives on the same thing. Like diff is the summary of changes in a commit. the person who wants to affect the change submits a pull request. Another person reviews the pull request and decides whether or not to merge their code into the main codebase, etc. It's all the same thing from different perspectives.
There is no "correct" name. You only see things from your standpoint, and so does everyone else. There is nothing about your opinion that is any less arbitrary than all the opinions that disagree with you.
When I hear “PR” I think of a tree of git commits. When I hear “diff” I think of one set of squashed text changes, for a tool that doesn’t support git.
