Churn

Churn is the loss of users, accounts or revenue over a period, counted when a customer stops meeting the product's definition of still being active.

Churn is the mirror of retention, but the two are exact complements only when they are computed on the same population and the same window. The word covers three different objects that get confused: user churn is losing people, account or logo churn is losing an organisation, and revenue churn is losing money, and revenue churn can even be negative when growth inside the remaining accounts outweighs what left. In a subscription product churn is observable, because a cancellation or non-renewal event exists. In a free or ad-funded product no such event exists, so churn has to be inferred from absence, which makes it a modelling choice rather than a recorded fact. The frequent error is comparing churn figures across products or periods without checking that the window, the population and the object being counted are the same.