I think this is the best description, but with a qualification. No mater when you joined the company, if you took no salary and got 'significant' equity, then you could be called a co-founder, especially you helped the company get off the ground or make a significant pivot - like Elon Musk for Tesla.

I know folks who are on Founder Board of Advisors. They work/provide guidance for sub-1% equity and no salary. They would never be called 'co-founders'