An Application Programming Interface (API) is a concept that acts as a software intermediary or a contract of service, enabling two distinct software components to communicate with each other. The core problem it solved was allowing different systems to exchange data and functionality without needing to expose their internal workings. The foundational idea for a modular software library was developed in the 1940s by Maurice Wilkes and David Wheeler for the EDSAC computer. The term "Application Program Interface" first appeared in a paper at an AFIPS conference in 1968, initially describing how an application interacted with its computer system. The modern era of public APIs began around 2000 when companies like Salesforce and Amazon launched their own APIs to allow third-party developers to integrate with their services.
An API works on a client-server model using a request-response mechanism. The application requesting information, the client, sends an API call to the server's API endpoint, often using the HTTP protocol. The API processes the request and returns a structured response, typically in data formats like JSON or XML. The rules for structuring these requests and responses are defined in the API documentation.
The most popular architectural style for modern web APIs is REST (Representational State Transfer), which is a key concept an informed reader should know, alongside older styles like SOAP. Recently, the API landscape has seen significant change, with the rise of the API-first development approach, which prioritizes API design before application building. Furthermore, API security has been strengthened by standards like RFC 9700, which mandates best practices for OAuth 2.0. A major ongoing shift is the transformation of APIs to be directly consumable by AI models and autonomous agents, with new concepts like Model Context Protocol (MCP) servers emerging around 2026.