The specific moment when a new user realizes the core value of your product. Getting users to this moment quickly correlates directly with long-term retention.
Why it matters: Onboarding funnels should be optimized entirely around driving users to the Aha moment as fast as possible, removing all unnecessary friction.
In practice: For Facebook, it was 'connecting with 7 friends in 10 days'. For Slack, it was 'team sending 2,000 messages'.