This isn't me not knowing the definition of the world, it's more that so many people use it in so many different ways i've found online. What does it mean when a book has good prose? What about bad prose? What defines good prose to you, and what seperates it from good writing? Is the word specifically describing something good, or is it a word you add "good" or "bad" to to clarify what kind?
I tend to think of prose as that subset of writing which is qualitatively deliberate in the kind of way that poetry is.
There's a quote to the effect that "the essence of [writing] style is the correct words in the correct places" and, mmm, I think that's a way of talking about it that's unhelpful for fiction writers. A ficiton writer's style is idiolectic; it's about personally characteristic words (punctuation, rhythms,) in characteristic places.
If I talk about the prose of a piece, it's like taking time to talk about something made of fabrics in terms of its textures. You can talk about writing being "good" in a lot of senses; technical writing can be good in the sense of correct, informative, clear, concise; but you rarely want technical writing to have inherently noticeable texture. You want it low-friction. You do not want it to snag the eye, leave you lingering over delightful word choice.
Other writing doesn't have to be about putting the textures under your hands; it doesn't have to make a point of that kind of richness. You can use the qualities of excellent technical writing to convey, say, fiction; and it's not a fault to do so. But it can be about that, and it's one of those things that makes writers especially memorable.
Like most things, it can be done more or less proficiently; and some writers' aesthetic sense is going to fall flat with me. I'd call writing prose if it seems to me that it's been crafted with the intent that its texture is an inherent part of the intended reading experience; and I'd judge it good if I think I can see what it's attempting with that, and whether I think it succeeded.
