A runtime switch that turns a piece of functionality on or off for some or all users without requiring a new deployment.
Example: Turning a risky new feature off instantly without a new deployment.
In practice: Flags let you decouple deploying code from releasing a feature, which is also what enables safe canary rollouts and instant kill-switches for anything that misbehaves.
Debated: Long-lived flags are widely seen as technical debt themselves — if a flag never gets cleaned up after full rollout, the codebase ends up permanently branching on dead configuration.