Glossary

Feature flag

A configuration switch that turns a piece of functionality on or off, or for specific users, without deploying new code.

Also called: feature toggle, feature switch

A feature flag is a conditional switch in code that controls whether a piece of functionality is active, checked at runtime rather than requiring a new deployment to change. The same flag can be set globally, on or off for everyone, or targeted, on for a specific percentage of users, a beta group, or a single account, using values read from a configuration service rather than hardcoded logic.

Flags are the underlying mechanism most modern A/B testing platforms use to assign users to a variant: a flag targeting rule splits traffic between "on" and "off," or among several variant values, and the experimentation layer measures the outcome difference. The same infrastructure also supports gradual rollouts, shipping a feature to 1% of users, then 10%, then everyone, while watching guardrail metrics, and instant kill switches, turning a feature off immediately if it causes problems, without a code rollback.

Feature flags matter operationally because they decouple deploying code from releasing a feature to users, which reduces the risk of any single release and enables experimentation without separate infrastructure. The common pitfall is flag debt: flags left in code long after a decision has been made, accumulating until the codebase carries paths for variants nobody uses anymore and nobody remembers why they exist.

Last reviewed September 22, 2026

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Related terms

A/B testing A controlled experiment that compares two variants to measure which one produces a better outcome. Holdout group A segment of users deliberately kept out of a treatment, campaign or feature, so its true incremental impact can be measured against them. Multi-armed bandit (MAB) An adaptive experiment that shifts traffic toward better-performing variants in real time, instead of splitting it evenly until a fixed end date. Feature adoption The share of eligible users who take up and continue using a specific feature after its release. Guardrail metric A secondary metric watched during an experiment to catch harm the primary metric would not show, such as load time or churn.

Related tools

Windmill Open-source developer platform that turns scripts into APIs, workflows and internal UIs, an alternative to Retool plus Airflow.

Related guides

How to choose an experimentation and feature-flag platform Pick by where your experiment data comes from — flag telemetry or your own warehouse — and which statistical engine your team trusts, not feature count. How to set up an experimentation program From your first A/B test to a durable program — the plumbing, the statistics rules, the review ritual, and the tools that support each stage.