API
An API is a set of features and rules inside a piece of software that lets other software interact with it, like a contract between the two.
One program asks, following the rules, and the other answers or does the task. The caller does not need to know how the other program works inside.
Why it matters for a small business
When a booking tool, your calendar and a WhatsApp assistant work together, they talk to each other through APIs of this kind. Ask any vendor which systems they connect to.
Source: MDN, 'API - Glossary'
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