Product-Market Fit
PMF
The point at which your product satisfies a strong market demand — the most important milestone for any startup.
Definition
Product-market fit (PMF) is the degree to which a product satisfies a strong market demand. Marc Andreessen coined the term to describe the moment 'when the customers are buying the product just as fast as you can make it.' Sean Ellis's survey method operationalizes PMF: if ≥40% of users say they would be 'very disappointed' if your product disappeared, you likely have PMF. Before PMF, a startup's primary job is to find it; after PMF, the job is to scale it.
Why It Matters
PMF is the most important milestone for an early-stage startup. Without it, pouring marketing spend and headcount into growth produces leaky-bucket results. With it, growth compounds naturally. Premature scaling before PMF is one of the most common and costly startup mistakes.
Example
Slack's earliest users described feeling physically ill at the idea of going back to email — a clear PMF signal.