Churn
Churn is the loss of customers or recurring revenue over a period, through cancellation or non-renewal, usually expressed as a rate.
In practice
Churn is the result that Customer Success, product and Sales qualification all feed into. Reported as a single number, it hides whether the losses are small accounts or large ones, and which reasons recur.
Customer churn rate = customers lost in the period ÷ customers at the start of the period. Revenue churn rate = recurring revenue lost from cancellations (and, if you choose, downgrades) ÷ starting recurring revenue. Record a churn reason on every lost account.