The share of customers or subscriptions that stop during a given period, such as a month or a year.
Churn measures how many customers you lose. If you start the month with 100 subscribers and 5 cancel, monthly churn is 5%. It can be measured by customer count or by lost revenue, and both are useful. In free apps, the equivalent is users who stop coming back.
High churn is like a leaking bucket: however many new customers you pour in, many drain away. Reducing it is often cheaper than acquiring new customers, and it reveals problems with the product, experience or price. It is one of the key metrics for subscription businesses. Practical tip: ask cancelling customers why with one short optional question, and separate deliberate cancellations from failed payments. Watch for warning behavior before cancellation, such as declining usage, so you can step in early with a message or help.