No worries, nothing wrong with not knowing everything about every random subject. I would like to apologize for being overly harsh, I assumed that people in c/opensource would have general knowledge of this definition, but that assumption was clearly bad. So again, sorry.

I assume the term is misused often.

Yes, companies sometimes do that. Open source is marketable as a guarantee that you won't fully lose access to a piece of software, and there aren't any real consequences of misusing it. But there's also a scheme called dual licensing where the software is available under two licenses - one license is open source but annoying for commercial use, and the other is a "normal" proprietary license under which businesses can buy the code. This is fine (as long as the provider has copyright to all the code being dual licensed) and is pretty common and makes the software open source.