The degree to which a product satisfies a strong market demand. Often characterized by high retention, organic word-of-mouth growth, and usage outpacing infrastructure.
Why it matters: PMF is the holy grail. Before PMF, the only goal is achieving it. Scaling sales and marketing before achieving PMF leads to high churn and wasted capital.
In practice: Marc Andreessen defines it as 'the customers are buying the product just as fast as you can make it.' A flat retention curve (users stay forever) is the mathematical proof.