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'

I like that distinction. Elon Musk is commonly thought of as a founder of Tesla, but he didn't actually join the team until he led their Series A round and became part of the board of directors.

Ref: http://www.quora.com/Tesla-Motors-2/How-did-Elon-Musk-build-...

Good distinction. The equity stake of a "co-founder" should be >10% range in typical cases. Working for sweat equity of a couple points doesn't strike me as having influence. On the flipside, owning 10-20% of the company makes you more influential (assuming you're also in an operating role), even if you are not there at the start.Reasonable things to consider, IMHO.