Feature flag glossary

Plain-English definitions of the terms that come up when teams adopt feature flags — what each one means, and how the ideas relate.

Feature flag: A feature flag is a conditional in your code whose value is controlled from a remote dashboard, letting you turn functionality on or off without deploying new code.
Feature toggle: Feature toggle is another name for a feature flag — a runtime switch that turns functionality on or off without a code deploy. The terms are interchangeable.
Percentage rollout: A percentage rollout serves a feature to a defined share of users — chosen by a deterministic hash — so the same users stay in the cohort as you ramp the percentage up.
Kill switch: A kill switch is a feature flag used to disable functionality instantly in production — flip it off and every SDK stops running the risky code path within seconds, no redeploy required.
Dark launch: A dark launch ships a feature to production turned off or invisible to users, so the code runs in the live environment before anyone sees it — you reveal it later on your own schedule.
Canary release: A canary release exposes a new version to a small group of users first and watches its metrics before rolling it out to everyone — so problems surface on a fraction of traffic instead of the whole fleet.
Progressive delivery: Progressive delivery is the practice of releasing software gradually and reversibly — using feature flags, canary releases, and percentage rollouts to expand exposure step by step while watching metrics, instead of shipping to everyone at once.
A/B testing: A/B testing serves two variations to randomly assigned groups of users and measures which one performs better on a chosen metric — turning a product decision into a controlled experiment instead of a guess.
Experimentation: Experimentation is the practice of making product decisions from controlled experiments — running A/B and multivariate tests, measuring outcomes against a hypothesis, and shipping the version the data supports rather than the one with the loudest advocate.
Trunk-based development: Trunk-based development is a branching model where everyone commits small, frequent changes straight to a single shared branch — keeping integration continuous and avoiding the merge pain of long-lived feature branches.

More terms are added as the glossary grows. For deeper explanations, start with the feature flags concept guide.

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