Environment variable
also: environment variables, env var, env vars
A named setting given to your app by the environment it runs in, rather than written in the code. Each environment can supply a different value.
A named value, such as DATABASE_URL, supplied to a running program by the operating system or hosting platform. It's the standard way to pass configuration and secrets per environment, and is often loaded locally from a .env file.
Think of a note left at a reception desk for each shift. The instructions say "call the number on the note". The instructions never change, but each shift leaves its own number.
Agents use environment variables for things like database addresses and API keys. If one is missing in production, the app can break even though the code is identical to what worked on your laptop.
The deploy failed because DATABASE_URL isn't set in production. Add it in your hosting dashboard's environment variables and redeploy.
Read the email service key from an environment variable, not the code. List every environment variable I'll need to set in production.
Thinking that setting a value on your laptop, for example in a .env settings file, also sets it in production. Each environment needs its own values set separately.