Retention is the percentage of users or customers who continue to use a product or remain subscribed over a defined time window. Time windows are typically Day 1, Day 7, Day 30, Month 3, or Year 1, measured at cohort level and treated as the single most diagnostic indicator of product-market fit and long-term growth. It is the inverse measurement of churn, and it is the only top-of-funnel-independent metric in growth: an unhealthy retention curve cannot be fixed by adding more acquisition.
The standard way to look at retention is a cohort curve, plotting the percentage of a defined cohort (users who signed up the same week, month, or quarter) still active over time. A healthy retention curve flattens, signaling that the product ha...