A web API is an application programming interface (API) for either a web server or a web browser. As a web development concept, it can be related to a web application's client side (including any web frameworks being used). A server-side web API consists of one or more publicly exposed endpoints to a defined request–response message system, typically expressed in JSON or XML by means of an HTTP-based web server.

A server API (SAPI) is not considered a server-side web API, unless it is publicly accessible by a remote web application.

A web API is an application programming interface (API) that is designed to be accessed over the World Wide Web. It may be implemented on either the web server or the web browser side.

In the context of web development, web APIs commonly interact with the client-side of a web application (including those built using various web frameworks). A server-side web API typically exposes one or more public endpoints that accept and return standard request–response messages, most often formatted in JSON or XML, and communicated via the HTTP protocol.

A server API (SAPI) is not necessarily considered a web API unless it is publicly reachable by a remote client over HTTP or a related web transport mechanism.

A client-side web API is a programmatic interface to extend functionality within a web browser or other HTTP client. Originally these were most commonly in the form of native plug-in browser extensions however most newer ones target standardized JavaScript bindings.

The Mozilla Foundation created their WebAPI specification which is designed to help replace native mobile applications with HTML5 applications.

Google created their Native Client architecture which is designed to help replace insecure native plug-ins with secure native sandboxed extensions and applications. They have also made this portable by employing a modified LLVM AOT compiler.