Hi Peter,
i guess you could call it a second level; it does not have to go any deeper or be more complicated than that. It is comparable to tree behaviour, although indentation of the second level (which a tree control normally has) is not appropriate because of the accordion characteristics.
A better analogy for understanding our accordion usage would be several itemlists / menus stacked upon each other, where only one is on top. The menu you click gets on top, the others become covered. Now one would name the second level of items 'menu-items' instead.
So, following my earlier example: one would click Animals, resulting in Monkey, Snake and Tiger to appear and the contents of Cars and Fruits to hide.
Then one would click Monkey, Snake or Tiger to open a page about that topic.
A selected item would be a nice bonus though. Nice to illustrate the menu item the designed page relates to.