Environment Variable
In plain terms
A named value supplied to a program by its environment.
Going deeper
Environment variables pass configuration (like API endpoints or secrets) to applications without hardcoding, varying per environment. They externalize config.
Under the hood
A Twelve-Factor staple for config; convenient but secrets in env vars can leak via logs/process listings.
Examples
API_KEY, DATABASE_URL, .env files, config per environment, Docker env.
Think of it like...
Like sticky notes left for a program telling it which settings to use here.