Churn is the share of customers, or of revenue, that a subscription business loses in a period because people cancel or downgrade.
Customer churn counts accounts lost; revenue churn counts the money. They can differ a lot when a few large customers leave or many small ones do.
Some churn is unavoidable. High churn in the first month usually means the product was not what the page promised, or that people never reached the part that is useful.
Why it matters for a launch
A launch fills the top of the bucket. Churn is the hole in the bottom, and it decides whether a good launch week turns into a business or a spike.