Wikipedia Has Become a Science Reference Source Even Though Scientists Don't Cite it (sciencenews.org)
Bethany Brookshire, writing for Science News: Wikipedia is a gold mine for science fans, science bloggers and scientists alike. But even though scientists use Wikipedia, they don't tend to admit it. The site rarely ends up in a paper's citations as the source of, say, the history of the gut-brain axis or the chemical formula for polyvinyl chloride. But scientists are browsing Wikipedia just like everyone else. A recent analysis found that Wikipedia stays up-to-date on the latest research -- and vocabulary from those Wikipedia articles finds its way into scientific papers. The results don't just reveal the Wiki-habits of the ivory tower. They also show that the free, widely available information source is playing a role in research progress, especially in poorer countries.
You cite a particular revision of the page. For example, for PVC if you wanted an old revision: https://en.wikipedia.org/w/index.php?title=Polyvinyl_chloride&oldid=802948163. The history page exists for a reason.