Here is mine (for fiction):
1 star - truly terrible, little or no merits. not even so bad its good kind of bad. Will most likely drop the book. Whether we should even rate books we haven’t finished is another discussion. However, I am of the believe if a book has nothing going for it even though you’ve read 100 pages, it’s probably a 1 star.
2 star - it was ok. Enjoyed bits here and there. It did not feel like a waste of time, but would not read again.
3 star - this one is hard to define. it was better than ok, but not great. would probably not reread, but fairly enjoyed.
4 star - very enjoyable, would not mind rereading. maybe there were a few flaws here and there, but nothing detrimental.
5 star - this book made me think about it for days, weeks, maybe even months. I care about the characters like they’re my own family. I would want to reread it again and again. Although, there are some books I would not want to reread, maybe because it dealt with dark or violent topics, but I would still rate 5 star because of the impact it had on me. Did the book change me, did it make me want to reread it, those are pretty good criteria for me to see if the book deserves 5 stars.
Note, I have never really rated books until recently. I realize it helps me reflect on how I feel about the book, and helps me get closure and move on to the next book without any unnecessary lingering attachment. Unless its a 5 star of course, then I’ll think about it forever, haha. I don’t even have a goodreads or do reviews or any of that jazz, just thought it would be fun to discuss.
5 star: perfection, wouldn’t change a thing, can’t wait to tell everyone about
4 stars: liked it a lot
3 stars: it was average/ok/alright
2 stars: did not like it/not enjoyable, forced myself to finish
1 star: hate it, scourge on the face of the earth, who green lit this garbage?
I review about 80+ books a year on my blog and social media (I read more but only write reviews for the copies the publishers send me) and I’d say most books fall in the 3.5-4.5 range for me.