Retention
The proportion of customers or revenue that remains over time — the clearest evidence of product-market fit.
Retention measures how much of a cohort is still active after a given period. It is the inverse of churn and the single most predictive early-stage metric.
The shape matters more than any one number. A curve that declines then flattens shows a core of customers for whom the product genuinely works. One that decays toward zero means no such core exists, however good acquisition looks.
Retention should be measured by cohort — customers grouped by when they joined. Blended retention across all customers hides deterioration behind new-customer volume.