I think we, the pedants, have lost this particular battle -- alleged "prototypes" (which by definition should be rewritten from scratch!!!-) are in fact being "evolved" into (often half-baked "betas"), etc.
Even today, I've applauded at the smart attempt by a colleague of mine to recapture the concept, even if the term is a lost battle: he's setting up a way for proofs of concept small projects to be developed (and, if the concept does get proven, transferred to software engineers for real prototyping, then development).
The idea is that, in our department, we have many people who aren't (and aren't in fact supposed to be!-) software developers, but are very smart, computer savvy, and in daily contact with the reality "in the trenches" -- they are the ones who are most likely to smell an opportunity for some potential innovation which could have real impact once implemented as a "production-ready" software project. Salespeople, account managers, business analysts, technology managers -- at our company, they all often fit this description.
But they're NOT going to program in C++, hardly at all in Java, maybe in Python but miles away from "productionized" -- indeed they're far more likely to whip up a smart proof of concept in php, javascript, perl, bash, Excel+VBA, and sundry other "quick and dirty" technologies we don't even want to dream about productionizing and supporting forevermore!-)
So by calling their prototypes "proofs of concept", we hope to encourage them to embody their daring concepts in concrete form (vague natural-language blabberings and much waving of hands being least useful, and alien to the company's culture anyway;-) and yet sharply indicate that such projects, if promoted to exist among the software engineers' goals and priorities, DO have to be programmed from scratch -- the proof-of-concept serves, at best, as a good draft/sketch spec for what the engineers are aiming for, definitely NOT to be incrementally enriched, but redone from the root up!-).
It's early to say how well this idea works -- ask me in three months, when we evaluate the quarter's endeavors (right now, we're just providing a blueprint for them, hot on the heels of evaluating last quarter's department- and company-wise undertakings!-).