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A runtime switch that turns a feature on or off without redeploying the code.
You wrap new code in a conditional checked against a config or flag service, so you can ship it dark, enable it for 5% of users, or kill it instantly if it misbehaves — all decoupled from the deploy. This is what makes 'ship it' safe and powers gradual rollouts and A/B tests. Example: launch a new agent model behind a flag, watch the metrics, then ramp from 1% to 100%. The flip side is flag debt: stale flags become their own footgun if you never clean them up.
Plainly
Think of Feature Flag as the checklist that keeps the app open for real people. A runtime switch that turns a feature on or off without redeploying the code.
In practice
Use it when a change has to survive deploys, users, incidents, analytics, or billing reality. In practice, define the owner, input, output, and failure mode before you rely on it.