API (or Application Programming Interface) refers to a set of rules (functions, variables, classes, protocols, etc.) that specific systems (applications, libraries) use to communicate with each other.
The API documentation is a technical content deliverable, containing instructions about how to effectively use and integrate with an API.
The basic requirements for API documentation are the same as for any piece of technical documents: it should be easy to learn, easy to follow, hard to misuse and always be up-to-date.
Programmers often generate API documentation with different tools, but this approach has its cons against professional technical writing solutions. Many programmers think that auto-generated docs are sufficient, but in most cases, it’s not true. Advanced solutions for technical documentation allow formatting, designing your code snippets and the whole look of the docs, creating advanced navigation and using search as well as many other opportunities. You don’t need to create a website or a WordPress page with API docs – many modern content management systems offer a hosted online documentation portal.