Glossary
Feature Flag
A feature flag is a switch in code that turns a feature on or off at runtime, letting teams deploy code without immediately exposing the feature to all users.
Feature flags, also called feature toggles, decouple deploying code from releasing functionality. The code for a new feature can ship to production hidden behind a flag, then be enabled gradually, for specific users, or all at once, without another deployment. This makes it possible to test in production safely and to roll a feature back instantly by flipping the switch.
Common uses include gradual rollouts to a small percentage of users, A/B testing of variations, granting early access to beta customers, and providing a fast kill switch if a feature misbehaves. Because the feature is controlled at runtime, teams can react in seconds rather than waiting for a new build.
Feature flags complicate the question of when something is 'released'. A change may be deployed for weeks before it is enabled, so release notes are usually written for the moment the feature is turned on for users, not the moment the code merged. Stale flags should be cleaned up once a feature is fully launched to avoid accumulating dead branches.