Activation is the funnel stage where a new user reaches first meaningful value from a product, the moment commonly called the aha moment. It is measured as the percentage of signups who complete a defined activation event within a specified time window (for example, "imported a first contact within 24 hours of signup," "sent a first message within 7 days," "invited a teammate within 14 days"). It is the stage in the AARRR funnel between acquisition and retention, and the single most-underinvested stage in most early-stage startups.
The classic framework for defining activation is the "aha moment formula" popularized at Facebook: identify the small set of in-product actions that, when completed early, predict long-term retention, ...