A feature flag (also called a feature toggle) is a code-level toggle that lets engineering teams turn features on or off in production without redeploying. It is used for gradual rollouts, A/B tests, kill switches in case of incidents, and entitlement gating between paid plans, and is now a near-universal pattern in modern continuous-deployment environments. The concept was formalized in Martin Fowler's writing in the early 2010s and operationalized by platforms like LaunchDarkly, Split.io, Optimizely, and the open-source Unleash and Flagsmith.
The four common use cases: gradual rollouts (ship a feature to 1 percent of users, then 10, then 50, then 100, watching metrics at each step), A/B tests (route 50 percent of users to var...