Churn Rate is the loss of future revenue due to the loss of a customer/subscription. There are two types of churn: Gross Churn and Net Revenue Churn.
Gross Churn: MRR lost in a given month/MRR at the beginning of the month
Net Churn: (MRR lost minus MRR from upsells) in a given month/MRR at the beginning of the month
Our churn rate has dropped considerably ever since we hired that fancy account manager.