Also known as Customer Churn Rate or Customer Turnover Rate. The rate at which customers are lost, generally expressed as a number or percentage of end users that defect within a given time period – usually weekly, monthly, or annually. Consideration of attrition rates is especially important to vendors and partners with recurring revenue models; for those companies, attrition rates, often referring to the number or percentage of subscribers to a service that discontinue their subscriptions in a given timer period – must be kept lower than retention rates to achieve profitability.