API (Application Programming Interface)

An API is like a waiter in a restaurant: you (one program) order something, the waiter (the API) takes the request to the kitchen (another program) and brings back the result. You don't need to know how the kitchen works - you just need to know what to ask for and what comes back.

APIs are everywhere, even if you don't see them: when a website shows a map, calculates a courier's shipping cost or accepts a card payment, it's using another service's API. They are the common language software uses to talk to each other.

For a company, APIs are what let systems exchange information in real time instead of exporting and re-importing files by hand. They are the technical foundation of almost every modern integration.