The most useful definition of activation starts with customer value. A sign-up tells you someone arrived. It does not tell you they found a reason to stay.
Choose a meaningful behaviour.
A trading product, a travel platform and a business application have different value moments. The first funded action, a completed booking or a useful workflow may matter more than an account created. Start by asking what the customer came to accomplish, and which observable behaviour shows they have begun to accomplish it.
Make it a shared operating priority.
Once that behaviour is clear, product, onboarding, marketing and customer support can work toward the same outcome. Map the steps before it. Identify the friction, uncertainty and missing information at each step. A growth campaign cannot compensate indefinitely for a journey that makes value hard to reach.
Measure what follows.
A useful activation event should connect to continued value. Look at what happens after it: repeat activity, retained customers, quality of participation and commercial contribution. Review the definition as the product evolves rather than treating the first measurement choice as permanent.