Retention is the share of users or customers who keep using a product over time. It's the opposite of churn.
It's often shown as cohorts: of the users who signed up in a given week, how many were still active one, four and twelve weeks later.
A retention curve that flattens out, rather than sliding to zero, is one of the clearest signs of product-market fit.
Why it matters for a launch
Growth from launches only compounds if users stay. Without retention, every month starts from scratch.