I have encountered something similar to this for a submission that I reviewed for a scientific journal. I will not list any names or give much detail past those generalities, but I pointed out that the authors were misusing a particular technical term. In my review I defined the term and explained it briefly. I asked the authors to revise their submission accordingly. The paper was not bad but the authors did not kno…
In your review, did you suggest the definition and explanation that they used? In this situation, would have an acknowledgment at the end have been enough? In my mind, it seems like you all had a conversation and the authors took up your suggestions as the reviewer.
Once I re-read the submission I wanted to reject it immediately, but I realized that I should get a second opinion first. So I contacted the editors, who agreed that it was blatant plagiarism. Hence, they rejected the paper once I recommended rejection in my second review. So this wasn't just a conversation where I made some suggestions and the authors used them. Even the editors thought it was plagiarism once they looked at it.
An acknowledgment would be impossible because the review was single-blind. The reviewers knew the identities of the authors but not the other way around. What the authors should have done was just re-phrase where they used the term in the paper. They didn't even need to copy my explanation, to be frank. The paper would worked fine without the paragraph they copied. If they just re-phrased the relevant parts no other changes would have been needed and this whole thing could have been avoided.