An agreed set of rules by which one piece of software talks to another. Short for application programming interface, and in print since 1968.
What it really means
An API is an agreement between two pieces of software: here are the questions you may ask me, here is the shape of the answer.
It stands for application programming interface, and the thing most people get wrong about it is the age. This is not a word from the era of web services. The term appears in print in 1968, in a paper on remote computer graphics given at an AFIPS conference, describing how a program interacted with the rest of the computer system. By 1974 it turns up in database literature in its modern spelling and enters the ANSI/SPARC framework. The idea is older still: Wilkes and Wheeler were building a catalogued library of reusable subroutines for EDSAC in the 1940s, which is the same instinct without the name.
The useful thing to understand is what an API actually is, because it is not a program. It is a promise about the shape of a conversation. Everything behind it — the language, the database, the machines, the company — can be replaced without the promise changing, and everything attached to the front keeps working. That is the entire value. It is also why a company breaking an API is a serious act: thousands of things were built on the assurance that the questions would keep being answered the same way.
The closest human equivalent is a pidgin: a deliberately reduced language, agreed between parties who need to do business and do not need to understand each other completely.
Where it comes from
The middle word was 'program' in the earliest use and became 'programming' later.
Myths & misconceptions
APIs are a web thing, invented with the modern internet.
The term is from 1968 and described how a program talked to the rest of a computer system, long before the web. Web APIs are one recent application of a much older idea.
An API is a piece of software.
It is an agreement about how to ask. The software behind it can be replaced entirely without the API changing, and that is the point: the interface is a promise about the shape of the conversation, not the thing doing the work.
How it connects
- Pidginthe same problem solved by people rather than machines: an agreed, stripped-down way for two systems to do business.
Questions people ask
- What does API stand for?
- Application programming interface. In its first recorded use, in 1968, the middle word was 'program'.
- What is an API in simple terms?
- A fixed set of requests one program agrees to answer for another, and the rules for making them. It hides how the work is done, so the inside can change without breaking everything attached to it.