Wikipedia
Churn rate (sometimes called attrition rate), in its broadest sense, is a measure of the number of individuals or items moving out of a collective group over a specific period of time. It is one of two primary factors that determine the steady-state level of customers a business will support.
The term is used in many contexts, but is most widely applied in business with respect to a contractual customer base. For instance, it is an important factor for any business with a subscriber-based service model, including mobile telephone networks and pay TV operators. The term is also used to refer to participant turnover in peer-to-peer networks. Churn rate is an important input into customer lifetime value modeling, and can be part of a simulator used to measure ROI using marketing mix modeling.
The phrase is based on the English verb churn, meaning "to agitate or produce violent motion".