environment variable
A named setting the computer reads from outside your code, often used for secrets and app settings.
Think of a note taped to a machine that tells it how hot to run. An environment variable is a small named value the computer reads before or while your app runs.
It matters because keys and settings must not live in the app itself. You put things like the database URL, a web address for your data, and API keys, secret passes for services, into environment variables.
In practice you set them in a settings file on your computer or in a cloud dashboard for your build or server. Your code reads NAME=VALUE pairs at run time, so you can change behavior without editing code.