The abbreviation API stands for Application Programming Interface.
In the classical sense, API is an interface for data and commands exchange between components of software systems (modules, complexes, applications, etc.). Typically, the API provides programmers with access to already developed software for use in applications. Usually, along with the API description, for more effective use of software by programmers, a set of development tools (SDK - Software Development Kit) is freely distributed.
Using the API ensures that all external applications interacting with the software system through the API will receive predictable correct results in response to their requests, and the system will correctly perform the necessary functions or scripts.
For the personal computer operating system , for example, with the API framework, the system responds to pressing certain keys on the keyboard or mouse, for the smartphone - touching a certain point on the screen.
The API can describe the interface of the application's interaction with the system at various levels - from the highest level, at the level of complex commands and processes, to the lowest, at the level of bytes and electrical signals.
In a broader sense, the word API, especially recently, is used as a software system interface, available not only to the programmer, but also to the end user.
In particular, the API now includes the languages of search engine queries (Google, Yandex), and the query language for databases. With this interpretation, any programming language, or, more accurately, its syntax and description of the subroutine libraries can be assigned to the API. In the graphical editor or the design automation system, a set of tools (primitives) for drawing basic elements of the image (lines, geometric shapes, components of drawings, etc.) can be considered as an API.