Churn (churn rate) is the percentage of customers who left in a period. For example, starting a month with 100 customers and losing 5 gives a monthly churn of 5%. It's the opposite of retention.
Churn is a hidden risk: even if you acquire new customers, if old ones keep leaving, the business becomes a "leaky bucket." At high churn, CAC is never recouped because the customer leaves before reaching their LTV.