Obviously thay cannot stop you from misusing a term. I used to believe like you that the definition was more broad like you indicated. But the people who coined it had a narrower free software defintion in mind.

https://en.m.wikipedia.org/wiki/Open-source_software

The fight over the meaning of "Open source" was lost by OSI almost two decades ago.

Here's a 2002 version of their website essentially admitting as much: https://web.archive.org/web/20021001164015/http://www.openso...

>The OSI Certified mark is OSI's way of certifying that the license under which the software is distributed conforms to the OSD; the generic term "Open Source" cannot provide that assurance, but we still encourage use of the term "Open Source" to mean conformance to the OSD.

As I wrote, no one would likely try to legally prevent you from calling any proprietary license open source if you want to. It just won’t actually be open source license in the minds of most if it doesn’t comply anwith the OSI guidelines

What about code that complies with the OSI guidelines, but does not use an OSI-approved license? Would you say that is open source?

Not up to me to say what is open source. If something was compliant with OSI guidelines, it’s probably fair to call it open source. Also probably pretty ill-advised to have a one-off license no one else uses.