API
/eɪ piː aɪ/ · Abbreviation · Development
Definitions
API stands for Application Programming Interface. It is a defined contract that allows two pieces of software to communicate with each other by specifying what requests can be made, what data formats to use, and what responses to expect. APIs enable developers to build on existing services without needing to understand or access their internal code. Web APIs, which are accessed over HTTP, are the most common type today, powering integrations between services like payment processors, mapping platforms, and social media networks. APIs come in several architectural styles, including REST, GraphQL, gRPC, and SOAP, each with different tradeoffs around flexibility, performance, and ease of use. A well-designed API is versioned, documented, and consistent, making it easy for external or internal developers to integrate with. APIs are the backbone of modern software, enabling microservices architectures, mobile apps, third-party plugins, and the broader ecosystem of connected services.
In plain English: A set of rules that lets different software programs talk to each other — like a waiter taking your order to the kitchen and bringing back your food.
Informally, 'API' is used to describe any boundary or interface between systems, teams, or even people. A manager's 'API' is how you interact with them; a team's 'API' is its published contracts and communication channels.
Example: 'The design team's API is a Figma link and a 30-minute weekly sync. Don't DM individual designers with requests.'
Source: metaphorical / informal
Etymology
- 1968
- The concept of an application programming interface emerges in a paper by Ira Cotton and Frank Greatorex for a remote computer graphics session
- 1970s
- API becomes standard terminology in systems programming, initially referring to OS-level function libraries
- 2000
- Roy Fielding's PhD thesis defines REST, sparking the modern web API era
- 2006
- Twitter and Facebook launch public APIs, popularizing the API economy
Origin Story
A 1960s library metaphor that became the backbone of the internet
The term **Application Programming Interface** first appeared in a 1968 paper by Ira Cotton and Frank Greatorex titled *"Data Structures and Techniques for Remote Computer Graphics."* But the concept predates the name: since the earliest days of shared libraries and operating systems, programmers needed agreed-upon ways for software components to talk to each other.
For decades, API meant a local interface — function signatures in a library or system calls in an operating system. The revolution came with the web. In 2000, Roy Fielding's doctoral dissertation described **REST** (Representational State Transfer), and companies like Salesforce (2000), Amazon (2002), and Flickr (2004) began exposing their services as web APIs. Suddenly, APIs weren't just for code on the same machine — they connected services across the internet.
Today, APIs are the invisible plumbing of the digital economy. Every time you log in with Google, check the weather on your phone, or pay online, dozens of API calls fire behind the scenes. The modern internet is less a collection of websites and more a vast mesh of APIs talking to each other.
Coined by: Ira Cotton and Frank Greatorex
Context: AFIPS Conference paper, 1968
Fun fact: Twilio's CEO Jeff Lawson wrote a book called 'Ask Your Developer' arguing that APIs are as transformative as the printing press. Whether or not that's hyperbole, the global API economy is estimated at over $5 trillion.