Ultimately product-market fit means that a product addresses an important need better than any other known solution. Validating product/market fit is an important milestone in many new startups and begins a transition to aggressively scaling the business. For a product that solves a long-term need, the best proxy is user retention cohorts that plateau. This means that some percentage of users who try the product like it enough to keep using it over a long period of time. For low-frequency use products (like Airbnb) seeing a plateau can take quite a while to see and might prevent the team from confidently scaling the business. For these types of companies, a survey question I have developed can be very useful. It is simply to ask product users how they would feel if they could no longer use the product. Those who say that they would be “very disappointed” without the product give you an indication that it is the best solution that they have found for an important need. Another thing to consider is that some products are not long-term retention products such as a pet adoption marketplace or a dating app. For these types of products, it can be more useful to gather qualitative insights from users about how effectively the product helped them accomplish their goal.