Product-market fit
The degree of alignment between a product and the genuine needs of a sufficiently large market. It is achieved when customers adopt the product spontaneously, recommend it and would be disappointed to lose it. Before PMF, heavy investment in acquisition is counterproductive.
In practice
The most reliable PMF signal is cohort retention: if users from a given cohort remain active six months after acquisition without any re-engagement push, the product solves a real problem. Sean Ellis’s question — would you be very disappointed if this product disappeared? — with more than 40% answering yes is another valid proxy. Before PMF, growth is filling a leaking bucket: customers leave as fast as they arrive.