What is an API call? How is my API Usage measured in terms of API calls?

What is an API call? How is my API Usage measured in terms of API calls?

A document opened in Zoho Office editor (e.g., Writer) equals "1 API call". 

The API usage is measured on the number of "open document" requests sent to Zoho Office Integrator. We update the API call counter by 1 with every new "open document" request. 

For example, if you open 5 different Word documents (one at a time), it will be counted as 5 API calls. Similarly, a single Word document opened multiple times (say 5) will equal 5 API calls.

We do not count "Save" requests and other editor operations within the document as API calls.