The term co-founder is tricky. It implies that you were present at the inception of an idea. It suggests that you're the domain expert or the visionary. But what happens if you weren't in the room when the idea was birthed, or your role fills many of the other strengths required to succeed? Do you qualify to be known as a co-founder?
I've been asked this question before, and I often pause to understand the root source of the question. What is this title validating and why does it matter? Is it your equity, influence, or something else?
In my opinion, your contribution matters far more than the exact time you joined a venture or how much you shaped the initial vision. Tech companies are long lived, malleable ventures that need nurturing in many ways to take off. If you were there before a product, customers, team, or funding, you're worthy of the co-founder title regardless of you're equity split, domain expertise or absence during original ideation. What's most important is that you gravitate so intensely to the problem space that you make it your own and do everything in your power to bring the vision to life.
It doesn't have to be your idea for it to be your company!
How do you feel about this, and how do you delineate?