API is Application Program Interface, which is a set of routines, protocols, and tools for building software applications. A good API makes it easier to develop a program by providing all the building blocks. A programmer puts all these blocks together.
Most operating environments such as MS-Windows, most HIM/SCADA software, Historian Software etc, provide an API interface so that engineers/programmers can write applications consistent with the operating environment. Although APIs are designed for programmers, they are ultimately good for users because they guarantee that all programs using a common API will have similar interfaces. This makes it easier for users to learn new programs.
In engineering software, Application Programming Interface (API) is a set of software functions used by an application program as a means for providing access to a specific application or operating system's capabilities. Many OPC Servers offers API for user.