Retention
The percentage of customers who stick around. The inverse of churn, but usually tracked by cohort.
Retention is measured by cohort — you group customers by the month they signed up, then track what % are still active N months later. A retention curve that flattens (goes horizontal) signals product-market fit; a curve that keeps sloping down means you're leaking users no matter how many you acquire. Retention is the single most important metric for whether a SaaS business is fundamentally healthy. High-growth SaaS with bad retention is a treadmill; slow-growth SaaS with high retention is a compounding machine.
Related terms
Churn
The rate at which paying customers stop paying you. The silent killer of SaaS.
LTV (Lifetime Value)
The total revenue a customer generates before they churn. What you can spend to acquire them.
PMF (Product-Market Fit)
The point at which users pull the product from you instead of you pushing it to them.