Product-market fit is the milestone every startup chases and few can define. Here is a plain-English answer, the signals you have it, and how it differs from go-to-market fit.
Product-market fit (PMF) is the point at which a product satisfies a strong market demand, where customers want it, use it, keep using it, and tell others.
PMF is when the market pulls the product out of your hands. It shows up as strong retention, organic word of mouth, and customers who would be genuinely disappointed if the product disappeared. It is the prerequisite for scaling: without it, growth spend just accelerates churn. PMF is about the product wanting; go-to-market fit is the separate question of whether you can reach and win those customers repeatably.
PMF is the dividing line between searching and scaling. Before it, the job is to find the product the market wants. After it, the job is to build the engine to sell it, which is where a GTM strategy and a repeatable motion come in. Trying to scale before PMF is the most common and expensive startup mistake.