IMO both of these ended up being poor names.
“Open source” can be co-opted to mean any project with public source code even if it’s not open contribution (think SQLite, and many of the projects effectively run by major tech corporations).
“Free software” falls victim to the eternal mixup with freeware, requiring the endless repetition of the “beer vs. speech” analogy.
I personally think “Libre software” is the term that best encapsulates the intended meaning while being unambiguous and not vulnerable to misinterpretation.
FOSS is even worse. Free and Open Source makes it sound like free of charge and see my source code.
It’s also a politically neutered term and an acronym of conflated concepts.
FOSS is… Literally as you described it.
What is there to conflate? It’s pretty obvious.
English language…
Many other languages have different words for each type of free
And yet our professors at university translated “free software” using our word meaning “free of charge”, my ears bled. It should have been libre software from the beginning.