Definition
A cohort is a slice of users defined by something they have in common: signup week, plan tier, country, or a behavioural condition like "fired the activation event in the first session". Cohort analysis is how teams check whether retention is improving over time.
How drift breaks cohorts
Behavioural cohorts depend on the events they reference. If an event gets renamed or moved, the cohort silently shifts membership. A retention chart that used to track week-one-active users now tracks something else entirely.