In its broadest sense, churn rate is a measure of the number of individuals or items moving into or out of a collection over a specific period of time.
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. The phrase is based on the English idiom "to churn up", meaning to agitate or produce violent motion.