API

An API (application programming interface) is how systems talk to each other without a person in between. An online shop registers a parcel through the courier’s API; accounting software fetches a statement through the bank’s API.

An API defines what can be asked for, in what form, and what comes back. It also sets limits: who is allowed to call it, how many requests are permitted per minute, and what happens when something fails.

For a business the term itself matters less than three questions about a specific system: does it have an API, is it documented, and does it allow what you need? The answers decide whether an integration is simple or hard.