The first moment a new user crosses from "signed up" to "got real value," usually pinned to a specific in-product action taken inside a fixed time window.
Slack's growth team determined in 2014 that teams reaching about 2,000 messages sent retained at far higher rates than those that did not. That single number reshaped onboarding: import history from competitors, encourage early channel creation, prompt invitations on day one. The 2,000-message bar is still cited in B2B growth playbooks as the textbook example of a sharply defined activation event.