Do you want MY definition of prolific or what people on lists I've frequented tend to call it? I'll start with mine...
A prolific writer is one for whom the words flow much faster than average. If they're publishing, you'd HOPE those stories are good, but there's no guarantee of that, as we all know. After all, the definition of prolific is "producing abundant results/fruit/works." It never says that those have to be "good works," and some of the more prolific authors in history haven't been believed to be all that "good." (Keep in mind that good is one of those terms that is always subjective, so there can be a lot of arguement there.)
I might note that you can be prolific without publishing, but then who would KNOW you were prolific? Grinning.
Your "word count" isn't dependent on genre or type you're doing. Someone that writes a romance novel, six fantasy novellas, a dozen assorted short stories and a handful of non-fiction articles in a single year is still prolific, though it's not all of a type.
Yes, that's an opinion of mine. If you want to know what people on lists say, the consensus seems to be that if you write more than 200K of publishable work per year, you're prolific, and if you write more than 400K per year, you're very prolific.
Nor is being prolific a sign that the work will be lousy, despite the comment I made about some of the most-prolific authors on record. Some people write slowly and well, some quickly and well, some quickly and poorly...and some poor sap out there agonizes over every word, and it still stinks.
Why people are abundant varies. Some push themselves to abundance. Some just write that way. Just my opinion... I tend to feel that people whose words just flow that way are the cleaner, more powerful writers. The words come for you the way they come for you. If you force a change to that flow, it usually results in less powerful results, IMO. I don't mean shaking things up. I mean forcing an artificial change to your natural, comfortable writing style.
How to decide if someone is prolific? Well, if they're published, that's easier to gauge than if they're not. As I said...how would you know without that gauge? I suppose, if you were an editor/agent, you might be able to see it by sampling some of the copious books someone claims to have finished, but... That's another issue, isn't it?
I've got a good list of prolific authors (the 20 most-prolific in literary history). I'd definitely put Asimov and King up there, to boot...and both of them are on my respected list. But, there are a lot of prolific ones today I could point to. Sherrilyn Kenyon is very prolific. No, she's not on the list, but she's written a lot more than she's published and she's still young. Who knows how much she'll publish by the time she leaves us? I'm hoping that doesn't happen for a long time.
People call me prolific. I won't argue it. I average 50K of new work every month, plus marketing, edits, etc. In my most prolific months, I was doing 80-100K per month, which I never want to repeat...really. One of those is scheduled to come out in the next two months, and my crit partners think it's the most powerful thing I've ever written.