AARRR (Acquisition, Activation , Retention , Referral, Revenue), aka the Pirate Funnel : Dave McClure's five-stage model that pins one measurable metric to each stage of growth.
McClure, then at 500 Startups, presented "Startup Metrics for Pirates" arguing that founders drowned in vanity metrics because they had no shared funnel vocabulary. The five stages mapped to five questions: did they find you, did they try it, did they come back, did they tell friends, did they pay. The deck has been downloaded more than a million times and remains the most-cited growth framework in early-stage startups.