What is Feature Flag?
A runtime toggle that turns features on or off without deploying code, enabling gradual rollouts, A/B tests, and kill switches.
Definition
A feature flag wraps new functionality in a conditional evaluated at runtime, so behavior changes without a deploy. Flags decouple release from deployment: code ships dark, then gets enabled for internal users, 5% of traffic, or a specific customer segment. They power A/B experiments, entitlement gating, and kill switches that instantly disable a misbehaving feature. LaunchDarkly, Unleash, and Flagsmith provide targeting rules and audit trails; simple cases use environment variables or database-backed toggles. The main hazard is flag debt — stale toggles multiplying code paths — so mature teams schedule flag removal.