To move something, especially a liquid, with great force. (Cambridge Dictionary)
To mix milk until it becomes butter. (Cambridge Dictionary)
If your stomach is churning, you feel ill, usually because you are nervous. (Cambridge Dictionary)
A churn is a container which is used for making butter. (Collins Dictionary)
If something churns water, mud, or dust, it moves it about violently. (Collins Dictionary)
Churn is the single metric that determines the success of your subscription service.