API
Also known as: Application programming interface
An API is a defined interface that lets one piece of software request data or actions from another.
In full
An application programming interface is a published contract describing how one system can ask another for data or ask it to do something. On the web this usually means an HTTP endpoint that accepts a request and returns structured data, most often JSON. APIs are how a website checks stock in an inventory system, takes a payment through a gateway, creates a booking in a scheduling tool, generates a shipping label, or looks up a business in a public register. They allow specialised systems to be combined rather than rebuilt.
Why it matters
Almost every useful integration on a business website is an API integration. Whether a system you rely on has a usable API often determines whether a desired feature is a small job or an impossible one.