API
Etymology
Short for Application Programming Interface. "Interface" is the shared boundary where two systems meet and agree how to interact — here, the published set of requests one program will honor from another. APIs turned software into building blocks: instead of rebuilding a map or a payment system, you call someone else’s through its API.
noun · /AY-PEE-EYE/
A defined way for one piece of software to request services or data from another.
What it names
The invisible plumbing connecting apps — the reason a travel site can show flights from dozens of airlines it doesn't own.
Example
When the weather company’s API went down, every app that relied on it lost its forecast.
Related idea
An API is the contract between programs, whereas a user interface is the contract between a program and a person — one is buttons and screens, the other is structured requests machines send each other.