Yes, Application Programming Interface. Basically, Microsoft wrote a ton of subroutines and procedures and together they make up windows. For example, there is a subroutine that allows you to create a window. Another to create a button, or to get the current time, or to save a file, or whatever. The big stink is that Microsoft created some and did not tell anyone, so competitors claim that those undocumented API's gave them an edge. A company then finds an undocumented API, uses it, and in the next version of Windows, Microsoft takes it out and the program no longer works anymore.