Product-market fit is when a product has found an audience that needs it badly enough to use it despite its flaws, recommend it without being asked, and pay for it.
Marc Andreessen's description: "when you have it, you know." This is true but operationally useless. Better proxies:
- Cohort retention: do users who sign up in month 1 still use it in month 6?
- Organic growth: are users arriving without paid acquisition?
- "Pull" from the market: are people asking for features you haven't built yet?
You can build a product forever without knowing if you have PMF. These questions force the answer.