An open API, sometimes referred to as a public API, is an application program interface that provides a developer with programmatic access to a proprietary software application.
An API is a software intermediary that makes it possible for application programs to interact with each other and share data. It’s often an implementation of REST that exposes a specific software functionality while protecting the rest of the application.
Open APIs are published on the Internet and shared freely. A startup software company, for example might publish a series of APIs to encourage third-party developers in vertical industries to be innovative and figure out new ways to use the startup’s software product. In theory, it’s a win-win business arrangement.
The third-party developer can make money by licensing his new program, a mashup with advanced functionalities that would be almost impossible to create from scratch. The startup gets to expand their company’s user base without having to spend any money to develop niche industry software -- and they still get to keep their source code proprietary.
See also: middleware