The rate at which customers or revenue leaves your business. Lower is better.
Churn is the leakage. Customer churn = % of customers who cancel in a period. Revenue churn = % of revenue lost in a period (different from customer churn when customers downgrade). Track both, because they tell different stories. Losing one big customer and keeping many small ones looks fine by customer churn but terrible by revenue churn.
Churn is the tax on every growth effort. High churn eats new acquisition. Unchecked churn kills businesses quietly; you grow through a valley and don't notice until revenue flatlines. Measure monthly. Investigate every cancel.