I’ll restate. My descriptivist definition of the term “open-source”, was that something is “open-source” if you are free to do at least one of either forking the thing for your own use, or modifying the thing and pushing those modifications upstream. In other words, “open-source” software is software where you’re free to notice a flaw in the program and, by some mechanism, get it fixed in such a way that other people can benefit from the fix.
And yes, the OSI definition of “Open Source” is an organization enforcing an arbitrary definition, because it disagrees with many uses of the term “open source” — specifically, the OSI considers GPL-licensed software to not be capital-letters “Open Source”, despite everyone conventionally considering it regular-adjective “open-source”. (The FSF does a similar brain-damaged thing with the term “Free Software”, which has not-much to do with what people would call “free software” — but at least “free software” isn’t a very common term in modern parlance; people aren’t constantly talking about “free software” with the FSF then misconstruing their words as if they had said “Free Software.” Which is exactly what the OSI does when people talk about “open-source software”, and then the OSI misconstrues their statements as statements about “Open Source software.”)
Yes, this is exactly why pedants use the term “FOSS” — i.e. “Free and Open Source Software”, a union-ing of the set of software that is FSF-jargon “Free Software” with the set of software that is OSI-jargon “Open Source Software.”
Regular people, who aren’t trying to be sycophants toward these two organizations, on the other hand, just use the uncapitalized adjective open-source to refer to this larger set.