User Churn is the number of users who stop using a product over a specific time frame. Churn rate can be calculated relatively simply, for example:

If you start the year with 100 customers and end the year with 80, your churn rate is 20% of customers. 

Losing some customers is inevitable; however, customer acquisition can cost five times more than customer retention. Therefore, keeping the churn rate low improves profits. User onboarding is one of the areas most subject to customer churn. Effective engagement is vital.