[DeadBabyOrgasm]

Environment variables with fallback to a default value.

They're a super cheap way of

1. allowing feature flags

2. injecting credentials in a way the user thinks about exactly once

3. moving workstation-specific details out of your code repository

They're implemented into the core of most every language in existence (especially shell scripts) and you're probably already using them without knowing. They're (get this) _variables_ for tuning to your _environment_.

Sounds like I'm being sarcastic here (eh, maybe a bit) but it never really hit me until I really dug into the concept.