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A runtime setting passed from the host environment instead of hard-coded into source code.
Environment variables hold values that change by environment: API keys, database URLs, feature flags, app URLs, and deployment settings. They let the same code run locally, in preview, and in production with different configuration.
Plainly
Think of Environment Variable as the checklist that keeps the app open for real people. A runtime setting passed from the host environment instead of hard-coded into source 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.