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.
Stale flag: A stale flag is a feature flag that no longer makes a live decision — it serves one variation to everyone, receives no traffic, or was never used — yet still exists as a conditional branch in code.
Flag debt: Flag debt is the accumulated maintenance burden of feature flags that have outlived their purpose but still sit in the codebase as conditional branches nobody has removed.
Multivariate testing: Multivariate testing compares more than two variations at once — often combinations of several changes — to find which performs best, extending an A/B test beyond a single either/or split.
Segment targeting: Segment targeting serves a flag variation to a named, reusable group of users defined by shared attributes — like "beta users" or "EU customers" — instead of listing individuals one by one.
Flag evaluation: Flag evaluation is the process an SDK runs to decide which variation a feature flag returns for a given user — applying prerequisites, targeting rules, and rollouts to the user context.
Targeted rollout: A targeted rollout releases a feature to a specific audience — defined by user attributes or a segment — rather than to a random percentage of everyone, so the right users get it first.
Feature management: Feature management is the practice, and the category of tools, for running feature flags as a system: defining, targeting, rolling out, and retiring them from one place without redeploying.
Remote config: Remote config stores application settings on a server so you can change them without shipping a release, using the same delivery mechanism a feature flag does but for values like limits, copy, and thresholds rather than on/off decisions.
Variation: A variation is one of the possible values a feature flag can return — true or false for a boolean flag, or one of several strings, numbers, or JSON objects for a multivariate flag.
Sticky bucketing: Sticky bucketing assigns each user to a flag variation with a deterministic hash of their identifier, so the same user always lands in the same bucket and stays there even as you raise a rollout percentage.
Blue-green deployment: A blue-green deployment runs two identical production environments — one live, one holding the new release — and switches all traffic over at once, with an instant switch back if the new version misbehaves.

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

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