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What is an API?

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Engineering Notes · AI Systems

The bank exposes account balances through a documented API rather than letting partners touch the database.

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Overview

An API, short for application programming interface, is a set of rules that lets different software applications talk to each other and exchange data. One program asks in an agreed format and the other answers in an agreed format, and neither needs to know how the other works inside. Almost everything online that talks to anything else is doing it through an API.
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Overview

An API is how one program asks another for something without either of them poking around in the other's business. Ask in the shape they want, get back what they promised, and stay out of it. Basically the entire internet is held together by this one arrangement, repeated a few billion times a second. 😎

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Detail

An API is a published way for one piece of software to ask another to do something: send a request in this format, and get a response in that format. A weather service might promise that if you send a city name, it returns today's forecast. The app asking never needs to know how that forecast was produced, only how to ask for it. That separation is the whole point, because it lets both sides change independently. The weather service can rebuild its entire system without breaking every app that uses it, as long as the API stays the same. The owner also decides which requests are allowed, who can make them, and how often. If those rules change in incompatible ways then old apps stop working, so companies usually release a new version rather than altering the old one. Modern AI systems lean on APIs constantly, whether they are checking the weather, reading your calendar or booking a flight.
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Detail

Every app on your phone spends its day begging other programs for things it cannot do itself. A flight-price site owns no planes; it asks each airline for today's fares, stitches the replies together, and hands you a tidy list as though it did something clever. The airline decides which queries it will entertain, from whom, and how many before it stops picking up. Nobody hands anyone their database. Ask in the published shape, receive the promised shape, stay out of the kitchen. And when the shape has to change, nobody edits the live one, because that snaps every app relying on it at the same instant. They publish a second version and let the first one retire with dignity. 😎

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Analogy

A restaurant menu. It lists exactly what you can order and what arrives if you do, and you never see the kitchen, meet the chef or learn the recipe. The kitchen can replace its ovens or change suppliers without telling you, and your order still works, because the menu is the agreement. Ask for something not on the menu and the answer is simply no.
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Analogy

A vending machine. You press B4, crisps come out, and at no stage do you get to climb inside and have a rummage. They can change supplier, rewire the lot, restock it upside down, and B4 still means crisps. Press a button that does not exist and you get nothing at all: no crisps, no explanation, no refund.
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AI explanations may contain errors · Not professional advice

Formal definition — The same term, explained the usual way

An API, or application programming interface, is a defined contract specifying the requests a software system accepts and the responses it returns, enabling interoperation without exposure of internal implementation. This decoupling permits independent evolution of provider and consumer, subject to versioning discipline, and provides the boundary at which authentication, authorization and rate limiting are enforced.

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