A runtime toggle that enables or disables a code path without redeploying, the safest way to ship risky changes.
A feature flag (also feature toggle) is a runtime conditional that enables or disables a code path based on configuration: a user ID, a percentage rollout, an environment, a kill-switch. Flags decouple deploy from release: code can ship to production behind an off flag, then be turned on for 1% of traffic, then 10%, then 100%, all without another deploy. The pattern is essential for canary deployments, A/B tests, kill switches, and gradual migrations.
Without flags, every code change is a deployment risk. With flags, the risk shifts to a config change you can revert in seconds. Mature SRE teams use kill-switch flags around every external dependency so a misbehaving downstream can be cut out of the request path without a rollback.
See the part of the platform that handles feature flag in production.