Login is necessary and almost never sufficient. Activation should name the first valuable outcome — publishing a board, inviting a teammate, completing a first analysis — and that outcome needs a reliable event.
In consumer and B2B apps alike across Korea, we see day-7 retention charts blamed for churn when activation never fired after a redesign. The product may be fine; the signal broke.
Write activation as a sentence a new hire understands. If the sentence needs three ands, split it. Then map one primary event and two supporting properties.
Revisit the definition when your core job changes. A metric that measured last year's product will punish this year's roadmap.