GitHub itself uses the OSI definition in its ReadME guide to open source:
> Many people think that Open Source simply means availability of the source code of a project, but that does only tell part of the whole story.
> The Open Source Initiative (OSI) provides a commonly accepted definition of what constitutes Open Source. To summarize that, in order to be constituted Open Source,
>> a work has to allow free redistribution,
>> the source code needs to be made available,
>> it must be possible to create further works based on it,
>> there must be no limitations of who may use the work or for what purpose (so something like "no commercial use" or "no military use" won't fly with Open Source),
>> the work must not require an additional license on top of the one it comes with,
>> and finally, the license must not depend on a specific distribution format, technology or presence of other works.
> So, you see, it goes way beyond "the source code is available", in fact, a whole lot more requirements are stated that must be fulfilled in order for a work to really be considered Open Source.
Also, GitHub's most starred repo is freeCodeCamp (359k stars - https://github.com/freeCodeCamp/freeCodeCamp). Linked right in the repo's README, freeCodeCamp defines open source as:
> Open Source Software is code that is publicly available for people to view, modify, and share.
And “what everyone else thinks” is mostly different from the OSI definition.