I'm not sure how to provide a non-tracking link to Amazon's help page, and I'm not saying they're the authority, but they put a lot of work into "Help Topics" "Amazon Community" "Customer Reviews" and they have a roughly one page essay/checklist defining a fake review.

To massively paraphrase and summarize, a fake review is one created for the author's personal gain, or its a review where the author falsified their assumed to be distant relationship to the seller.

Its well written, although in practice it's unenforced and as a shopper, Amazon reviews are almost useless.