API

An API is the interface one piece of software offers to another: a defined set of operations, with defined inputs and outputs, that a program can call without knowing anything about how the other side works internally. It is the contract that lets two systems be built and changed independently.

Practically, an API is what makes integration possible at all — your website talking to your accounting system, your app talking to a payment provider, a customer pulling their own data into their own tools. Where a vendor has no API, every exchange of data becomes a manual export or a fragile scraper that breaks when a page layout changes.

The quality of an API is mostly the quality of its contract: whether it is versioned, whether it is documented, whether it breaks quietly when the other side changes. Those properties determine the maintenance cost of every integration built on top of it, which is why they belong in a supplier evaluation rather than in the technical appendix.