“Open source” has long been used used to describe research of publicly available information. A quick search in google books shows references back to at least the 70s. This meaning is consistent with the “source available” usage today.

"Open source" -- meaning the source is available and you can make modifications. You know, precisely what was in use before the OSI came along.

Sure, that's a starting point. What is missing is who "you" is and what can be done with the modifications.

If I say "source is available and you can make modifications, but only if you have red hair and live in Brazil", is this open source?

I think the common-sense definition that a lot people use, especially people not deeply invested in the movement, is something along the lines of "the source is available for everyone to examine and make modifications to".

It's not as specific as the OSI definition of course, and it doesn't say anything about redistribution and such, but that's okay and it doesn't really need to be IMO. Some vagueness is okay, and even with the OSI definition there is a huge difference between something like MIT and AGPL, both of whom are "open source".

A large majority of software developers and related professions use OSI's definition in my experience. Even the ones apathetic or antipathetic to the movement. The elements of OSI's definition have practical consequences for them.

People talk about permissive and copyleft licenses when the distinction matters. Covering both doesn't make open source vague.

Alternative definitions restrict the freedom to run the software at all frequently. Never mind modifying it.

Yes, and butterfly is a fly made of butter. English words mean what people understand them to mean.

The only thing you are saying is that other people tried to use "open source" for something else and completely and utterly failed to convince people. Thus, today "open source" means what it means.

Ok, thanks, that's one. But you said there are "many others"? Many other clear definitions of "open source", you seemed to be saying.