Retention
The share of users who keep coming back over time. Often shown as a curve that should flatten (not hit zero).
Why it matters
A flattening retention curve is the clearest sign of product-market fit.
Related terms
Churn Rate
The percentage of customers (or revenue) you lose in a period. The silent killer of recurring businesses.
Growth Rate
How fast a metric (revenue, users) increases period over period, usually monthly.
Activation
The moment a new user first experiences real value (their 'aha'). Not just signing up — actually getting the point.