Teams often inherit activation definitions from whatever event was easiest to track. That creates a neat chart but a weak decision tool.
Start with the moment a user receives the product’s core value. Then identify observable behavior that is close enough to that moment to measure consistently. Compare candidate behaviors against later retention, but do not confuse correlation with causation.
The final definition should be understandable to product, marketing, and engineering. Document its window, eligibility rules, exclusions, and known blind spots. A metric earns its place when it changes what the team does next.