Aiee! (An Interactive Environment Engine!) is an interactive fiction engine. It is not to be confused with the browser I.E. (even if dealing with that browser causes you to use the exclamation "Aiee!"), nor is it connected with the American Institute of Electrical Engineers.
The file format for Aiee! games is XML-based, and uses the .xml extension (making it hard to distinguish them from the many other sorts of XML files).
Aiee! files have <adventure> as their top-level element and have <item> and <room> elements within it.