Churn rate
The percentage of customers (logo churn) or recurring revenue (revenue churn) lost over a period — the single most-watched measure of retention failure.
Churn rate measures loss over a period. Customer churn rate is the number of customers lost during the period divided by the number you started with; revenue churn rate applies the same idea to recurring revenue. The two can tell very different stories: losing ten small accounts and losing one large one might be the same revenue churn but wildly different logo churn, which is why mature teams track both rather than arguing about which is "correct".
The most common analytical mistake is underestimating compounding. A rate that sounds small monthly is large annually: three percent monthly churn removes roughly a third of the customer base over a year, because the losses stack month over month. The second mistake is treating churn as a lagging number to report rather than a leading process to manage — by the time churn shows up in the metric, the causes are months old. That is the argument for health scoring and early-warning signals: they move the team upstream of the number.