(This was my answer to a question on Quora)
A co-founder is someone who has equal skin in the game. I.e. all of them are working full-time in the venture, all of them are making the same sacrifices (e.g. taking no or low salary.. unless all founders agree that one needs to take some/less/more salary because expenses like children’s school fee, EMIs, etc. have to be met).
A co-founder is someone who accepts the emotional and intellectual responsibility to convert the idea into a business, and who commits to live through the challenges that any entrepreneurial journey faces.