A feature flag (feature toggle) is a runtime switch that decides whether a piece of functionality is active, letting you turn features on or off — for specific users, a percentage, or everyone — without shipping new code. It decouples deploy from release.
Worked example: ship a new checkout behind a flag enabled for 5% of users, watch the metrics, then ramp to 100% — or flip it off instantly if something breaks, no rollback deploy needed. Gotcha: flags accumulate into technical debt fast — a codebase full of stale toggles becomes a combinatorial testing nightmare — so treat them as temporary and remove each once its feature is fully rolled out or killed.