Retention is the measure of how well a service keeps its users over time - the share still active after a week, a month, a quarter. As the inverse of churn, it is the metric subscription economics ultimately turn on, since a retained subscriber keeps generating revenue while an acquired-then-lost one rarely pays back their acquisition cost.
Analysts study retention with cohort curves that show how each group of new users decays over time, looking for the point where the curve flattens (a stable, habitual base) versus keeps falling. Retention is driven by everything upstream - onboarding, content, discovery, and QoE - which is why it serves as the north-star outcome that engagement and quality metrics feed into.
