The rate at which existing customers stop paying, usually expressed per month.
Two kinds, and conflating them hides the problem. Customer churn counts accounts lost; revenue churn counts money lost, which can be very different if your large accounts behave differently from your small ones.
Net revenue retention accounts for existing customers spending more — a business can lose accounts every month and still grow revenue from the ones that stay.
The arithmetic that catches people out: 5% monthly churn is roughly half your customers gone in a year. At that rate, growth requires replacing half the base before adding anything. Early-stage churn is also noisy — with forty customers, two leaving is 5% and might mean nothing at all.
Project types where this term stops being vocabulary and starts being a decision you have to make.