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?
because of the way i write fiction, i tend to think of prose and dialogue as the two primary units of written storytelling. this isn't absolute, as dialogue can be part of the prose and vice versa, but generally that's the difference. of course, every writer has their strengths and weaknesses. dialogue comes very easily to me, where prose/exposition often does not. so my perspective here is naturally colored by that.
prose is place-setting, internal monologue, exposition, all the stuff that isn't characters talking to each other. what determines good prose, in my opinion, is to what extent the author imbues their prose with character. it's easy for narration to be faminously utilitarian (especially today, as fewer people read books written before the dominance of TV, and so tend to think of their stuff as visual media that's been temporarily compromised into a book, and the prose as rarely more than stage direction. i am very guilty of this myself).
good prose doesn't just set a scene, it sets a tone and a mood. it grounds you in a reality or gets you thinking about its rules or history. good prose is just as energetic as good dialogue, stops feeling like mandatory place-setting and becomes an essential vehicle of thematic conveyance. as someone who finds describing locations and appearances exceedingly tedious, i'm always impressed when a writer manages to do so in a way that keeps me rapt and attentive.
conventionally speaking, prose is where an author has the most leeway to press their hand without (unintentionally) breaking the reader's suspension of disbelief. it's much easier to clock author mouthpiece dialogue than it is author mouthpiece prose, in my experience. there's an elegance and restraint to good prose. it's where you can really gauge the maturity of an author's craft. this isn't to say good prose is necessarily brief! there are no universal prescriptions when it comes to writing.
the measure is the same as any art: what was the author trying to do, and how close did they come to achieving it? good prose is good when you read it and go "damn, that was some good prose."
