> For the complete documentation index, see [llms.txt](https://docs.serverlessapigateway.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.serverlessapigateway.com/glossary/a/api-application-programming-interface.md). # API (Application Programming Interface) An API (Application Programming Interface) is a set of defined rules, protocols, and tools that allows different software applications to communicate with each other. APIs specify how software components should interact, abstracting away the underlying implementation and exposing only the objects or actions a developer needs. APIs are fundamental to modern software architecture. They enable integration between disparate systems, allow third-party developers to build on top of existing platforms, and support the decomposition of monolithic applications into microservices. Common API styles include REST, GraphQL, and gRPC, each with different trade-offs around flexibility, performance, and ease of use. In the context of API management, an API gateway sits in front of APIs to handle cross-cutting concerns such as authentication, rate limiting, request routing, and response transformation. This allows backend services to focus on business logic while the gateway enforces policies consistently across all exposed endpoints. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.serverlessapigateway.com/glossary/a/api-application-programming-interface.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.